Here's FANG prototype 3. I made this much in mid-March and said very little about it publicly. A number of you have already seen this. What is shown is not complete, however, this prototype has served its purpose very well and I will not develop it further.

I had originally intended to finish it so that the Firefly turret is advanced with each trigger pull, but I recently decided to redesign the gun substantially to reduce length and weight to improve maneuverability (among other things). The same basic layout will be used in FANG prototype 4 with shorter barrels, a smaller air reservoir, stronger construction, and a number of new features like a pump.



A high-resolution photo is available here.

Presently I do not intend for FANG 4 to have an automatically advanced turret as I'd like to focus more on my other ideas as well as compare good manually advanced turrets against automatically advanced onces in reliability, ease of operation, and RoF. Manually advanced turrets are easier to make and their RoF may be very acceptable for most Nerf wars.

Petroleum jelly's been misclassified. While it's definitely bad for natural rubber and thus should be used with caution, synthetic rubbers and most plastics should handle it well. I used petroleum jelly to lubricate a large Buna-N piston cup in a PVC pipe several years ago with no ill effects.

Chemical compatibility/resistance charts are useful here. Buna-N rubber, a common O-ring rubber, is perfectly fine with petroleum jelly (in fact, this chart rates it better than aluminum, brass, and steel). This chart suggests that polycarbonate and ABS are perfectly fine with petroleum jelly and this chart says the same for PVC. I'd suggest doing a Google search to find some of these charts if there is any question of the lubricant's effect on the material.

Edit:

One additional point about petroleum jelly that I had failed to mention was that the ONLY reason I used it was because I needed a huge amount of lubricant. I was lubricating two 4 inch piston cups. Needless to say I'd be spraying for a while from my can of silicone lubricant.

Silicone lubricant would be a lot less messy so it's my suggestion for Nerf just like the OP.

If there are any concerns about the compatibility of the materials and the lubricant, again, I'd suggest doing some searches for chemical compatibility tables.

As with all material choices, "better" is relative. What do you want?

UHMW-PE is weak (has a relatively low tensile strength) and bendy (has a modulus of elasticity that is about 1/3 that of PC), but can absorb a very high amount of energy in impacts. That is the reason to use UHMW-PE. It's very difficult to fracture UHMW-PE. So if you don't want your side panels to fracture, UHMW-PE is one option. Note, however, that this is not the most elegant option.

However, you should take into account the lower strength and lower elastic modulus. When converting from PC to UHMW-PE, use thicker pieces of UHMW-PE. I'm using a 3/8" thick piece in FANG 4.

Also, due (probably) to the long polymer chains in this material, when machining you may find your cuts won't be as clean as they might be with a different material. The polymer chains will hang off the edges, giving a ragged look to the material. This can be cleaned up with sandpaper and/or a file.

Additionally, UHMW-PE is among the worst materials for a plunger rod because of the low tensile strength. If you use UHMW-PE as a plunger rod (with typical dimensions) the rod will creep (i.e. slowly and permanently stretch), assuming that the rod can handle the force and the tapped threads hold.

I'm also interested in making one of these set ups and with the loss of the centerflag regulators, I would love to know of a new regulator I can purchase. Or even just a new reliable source of the Doolie regulator.

The Palmer Stabilizer regulator I linked to should be better than the Doolie regulator. The Stabilizer can regulate down to 0 PSI so you won't need a second regulator unless the Stabilizer is inconsistent (I doubt it is).

The inline regulators should work too but as CaptainSlug said, it's bulkier. If that setup is cheaper I might consider it though.

After some digging I think I'm on the right track. I've found a former employee who's emailed the owner of CenterFlag at his personal email address so I might be able to get through now. With that being said I doubt these regulators are still being produced. Likely I'll get my money back and that'll be it.

PayPal eventually gave me all my money back so it worked out fine.

I've been waiting for CS to post about his inline setup to see if it'll save money. If not I'm just going to use a female Palmer Stabilizer with a low pressure kit.

If I don't get a response in the next day I'll order it.

Edit: I bought the Palmer Stabilizer anyway. It came out to $140 with the low pressure kit, mounting plate and an 1/8 inch NPT tube that I couldn't find on McMaster-Carr, which is pretty damned expensive but I figure if it works I'll take it. I'll keep everyone updated on how well this works for me. I'm working on a Nerf gun at the moment and might have something finished in the next few weeks.

Edit again: What I'm going to do follows the UberSoaker water gun. I didn't realize it at first but I basically bought the exact same parts.

First I'd like to thank CaptainSlug for making this guide. It saved me a lot of time to say it in short. There's one problem with the guide though.

The first regulator noted here can't be bought any longer. Centerflag Paintball Products has essentially ceased to exist. Two weeks ago I ordered a regulator, and after emails, phone calls, and online research, it doesn't seem that Centerflag exists any longer. The phone number given on the order confirmation goes to some foreign guy who has never heard of Centerflag. The phone number on their whois doesn't work. I did find a phone number for one of their employees and I left a message, but I'm still waiting for a call back. Luckily I used PayPal and am working to get my money back.

I'm still researching regulators but I'm leaning towards a Palmer Stabilizer at the moment. I'll update everyone when I do buy a regulator and get the system working.

I already said $9 was the shipping. I don't have it yet so I don't know how consistent it is, but I imagine with the optional low pressure spring it would be more consistent because there's a smaller range to regulate to. I have two low pressure regulators so if it's inconsistent I don't have to buy anything. I'll let you guys know how consistent it is after getting it and fooling around for a bit.

Hm. $140 for just a regulator seems a bit steep to me, because I would still have to buy an HPA tank which appear to be around 80+ dollars. I was hoping to get this set up bit for under 200. BUT i guess if this set up works I may have to look into it.

About $50 of that was optional parts and $9 was shipping. If you just want the regulator it'll be $79 before shipping.

The mounting bracket I got with it was about $30 alone. Perhaps I shouldn't have got it, but it looked perfect and there's no going back now. I'd suggest at least the $79 regulator I got with the $10 low pressure upgrade so you can't regulate higher than 250 PSI.

Extremely impressive work, as always. You've been very comprehensive so far, so I don't really have many questions about the gun, but I do still have a few questions.

I'd be interested in seeing your ballistics code as I'm pretty interested in that aspect of Nerf gun design. I'm under the impression that you only analyzed the exterior ballistics. Is that correct?

Also, why did you bother factoring in compressibility? No Nerf dart should be fired over about Mach 0.3. Assumption of incompressibility are perfectly accurate in my experience for exterior ballistics. Interior ballistics is a different story, however; the wave propagation process can be ignored for most Nerf guns but the compressibility effects on the properties of the gas can not be. (For some details of that, I have a very simplified non-dimensionalized model of the interior ballistics of a pneumatic gun described here. The springer model and any of my exterior ballistics stuff have not yet been posted.)

Three….it is impossible for a spring-plunger type gun to even fire a dart above Mach 1.0.

No, it's possible and it has been done before. A number of spring-plunger pellet guns fire in the supersonic range. Spring-plunger guns are actually a rather decent design for high speed ballistics because the adiabatic compression of the gas increases its temperature, which in turn increases the speed of sound, which in turn allows the gas to expand to refill the space behind the projectile faster, etc.

Very interesting. I don't have a particularly strong background in compressible fluid dynamics (blame it on being a mechanical engineer, not an aerospace engineer) so I might have some specific questions in the future for you.

I meant more that there is no use in the hobby for darts over about Mach 0.3, unless someone is out to cause harm.

What drag coefficient and dart mass did you use for your simulations?

A while back I posted an equation I "derived" via curve fitting to a simple exterior ballistic simulation. How well does this fit with your simulation? (I seem to have lost the papers I used to go from the non-dimensional version that was the actual result of the curve fit, but I can redo that later when I have some free time.)

Looks nice and compact.

Is the air chamber port of your QEV off at a slight angle? That port on my QEV is off slightly and it's somewhat noticeable. I'm wondering if it was just a defect in my valve. Yours appears straight.

Glad to be of assistance. I'll admit one of my motivations to post that was to get you back in gear. I don't reliably finish projects but I know when I've stumbled on something useful so I thought it'd be best to get that out before I forgot about it.

Dr. Nerf's trigger system is just about as good as it gets for an air pressure system. Sure, it's not mine, but I helped him build it. Fast opening times combined with a lot of initial air translates into excellent range. Another unjustly forgotten system. Some concepts received more notice than this actual finished product.

As for something I actually made (two years ago), my moving barrel idea is the most unusual way to fire. You can either operate it with the handle on the top or the handle on the bottom (the bottom being like a pump). It's still the only system I know that advances the dart and fires in one motion. I've been working on upgrading this with a more efficient valve like the one Dr. Nerf used.

The design has a return spring if you look at the pictures.

One change I suggested to Dr. Nerf would be to use rubber sheeting as opposed to O-rings for these types of seals. There's a reason that's what they use in regular pull valves. Seals much more easily. About half the work is necessary!

Nice post, RedFear. I was about to suggest posting your own thread, but seems you already have. I'd suggest changing your post to a link to that thread rather than updating both, as it's easier.

Also, as a note, if someone suggests that I investigate something and I'm slow, I'd be perfectly happy for someone else to jump in. I already have a lot on my plate.

Edit: So, I am being slow in general due to other obligations. I'm aiming for one new blog post a month now, and I'll mostly be working through my backlog. Feel free to post here with suggestions, still, as I'll take a look and others might be able to help as well.

Doom,
In your opinion, what is more important when trying to hopper an airgun, volume or pressure?

This depends on what you mean by "important". According to my theory, whatever configuration (pressure and volume) that gets you the right muzzle velocity will work.

With that being said, high pressures tend to have higher energy efficiencies, so I would bias my own designs towards that. Of course, there are other practical concerns (material strength and difficulty of pumping) that limit pressure.

Also, it's worth noting that my theory hasn't been tested experimentally. It fits the high speed video, sure, but I'd like to see more evidence.

If you want some starting points, there are some numbers in this thread, e.g.:

The only data I could find was CaptainSlug's Pepe, where he used a 10 psi 7 ci tank with a hopper.

Split at NerfHaven told me he couldn't get a hopper to work with a "small" 4 in^3 firing chamber.

I'd make a small tank, and then a tank an order of magnitude larger, and another 2 orders larger. Maintain vent hole size with each. Test each at various pressures with the same hopper and barrel system, with a fake barrel that's only a few inches long, in order to demonstrate chambering. Sound about right?

That's a great start. I'd also like to consider the effect of dart mass (to isolate how much gravity helps), barrel length, dead space, dart head shape, number of darts in hopper, possibly the effect of some of the changes people have made to the feed path, and possibly firing angle (right side up, upside down, sideways). That's a lot of variables, so I'd like to get some idea about which are important.

Given that, I made a simple mathematical model of a hopper. I'll detail this in another post. It helped clear up what the likely important parameters are in my mind. To summarize, it seems that one of the major issues with hoppers is the muzzle velocity. I hypothesize that there's a muzzle velocity below which hoppers won't work. Let's call this the "critical hopper velocity" to sound scientific. This muzzle velocity seems to be controlled by the details of the hopper and darts, so it's not a universal constant. But, perhaps it's similar for most hoppers.

So, if I were to do some tests, I'd start with an airgun and a chrono. Barrel, chamber volume, number of darts in the hopper, etc. are constant between tests. The pressure is varied to change the muzzle velocity. If my hypothesis is true, at "low" pressures hoppers won't work, at pressures near the critical value the hopper will work about half the time, and at "high" pressures the hopper will work reliably.

Now, as for the question of what pressure and volume will get your gun to that velocity, that's where a computer simulator like GGDT comes in. Or, alternatively, experiments and past experience.

One of the other issues is muzzle blast, that is, blowing air out of the barrel after the dart has left. I hypothesize that hoppers require muzzle blast to work. The muzzle blast is likely what advances the hopper. This means that your barrels must be shorter than the ideal barrel length to use a hopper, which reduces the muzzle velocity. Muzzle blast in combination with the added dead space probably account for most of the energy loss associated with hopper use. Experiments and simulators can help you design a gun with a certain amount of muzzle blast. (I'm not sure how long a duration is necessary, though I'll do the analysis for it and compare that against the high speed video I have. First step is to figure out the frame rate of the video.)

I have no idea how many variables would need to be accounted for in order to solve for barrel length, but it seems logical to say that the best barrel length would be found where the acceleration of the dart equals zero. This would correlate with the maximum muzzle velocity of the dart. If there was a way to track the position of the dart inside of the barrel, it would be easy enough to make a function from that data, find the second derivative of that function and solve for y=0.

I'm not entirely sure if any of this is exactly practical, or if it is the best way to solve this problem. I would love to hear what you think about the subject, and hopefully the community will eventually find a use for the fruits of this labor.

Yes, maximum velocity is the criteria. You can read the internal ballistics section of my old notes for two ways I estimate this theoretically. The problem with theory here is that the equations can't be solved exactly in the most general case, so you are limited to simplified cases. You can get around that by using computer simulation techniques, but in that case it's harder to understand precisely what controls what. The theory helps you understand why and the computer simulation is more practical. Indeed, spud gunners use GGDT all the time to find ideal barrel lengths.

I don't know anyone in the Nerf or spud gun communities who have posted any useful data about experimentally finding the ideal barrel length. The easiest way to test this is to start with a barrel that's too long, shoot the gun while measuring its muzzle velocity (multiple times, because there'll be variability), and cut the barrel down in increments. Repeat this until you have an idea of what length works best. Despite the fact that this method has been suggested many times, I am unaware of anyone trying it.

An interesting alternative is to use a magnetic projectile and put coils on the barrel. From that you can back out the velocity at different points in the barrel and minimize the number of tests you need to do, because you can do one test with a long barrel. (The actual ideal length will vary a bit from what you'll find in this technique due to the flow being constrained by the long barrel, but this is really only important when you Mach number is high. I hope no one has that issue in Nerf.)

I know you already have your plate pretty full, but could you possibly make an article(or two) detailing the pressure and/or air volume required to hopper an airgun?

This was part of the plan. The problem is that we don't really have any good test data for this. Someone with more time on their hands could easily beat me to it. I could detail how I'd test this if anyone wanted to do it.

I would say that coefficients of friction for various brands of FBR on various brands of CPVC would be a good idea, but there is probably far too much inconsistency for useable data to be accumulated. Perhaps some common match ups could be outlined if a universal barrel length formula is ever created.

Friction measurements are one thing we seriously lack. The problem in Nerf is actually worse than you might imagine. The friction force is a function of the coefficient of friction and the normal force. Neither is known. Roughly, what we do know is that tighter barrels will have higher normal force and that some materials have less friction than others.

I recall discussing some ways to estimate the friction force with Split a while back. He used the bulk modulus to estimate the normal force. I suggested something more complicated that probably would not be much more accurate than his approach. I should look more closely into this.

Spud gunners use a simulator called GGDT which allows you to specify the total friction pressure, which I call the pressure of friction. This is basically the total friction force divided by the surface area. It represents how high the pressure needs to rise for the projectile to move and In GGDT, static and dynamic friction are equal, which is not realistic or necessary, so I don't make this simplification. I stole this notation for my own work.

If you can blow a dart down a barrel, you can be sure that the static pressure of friction is lower than the maximum pressure your lungs can generate, which seems to be about 0.9 to 1.4 psi for men.

As for ideal barrel length, I'll make a thread detailing two simplified theories I developed, how to measure this experimentally, and how to estimate this via a computer simulator. My simplified theories give very approximate equations which can be useful to get a rough estimate of the actual ideal barrel length and understand the basic factors involved in ideal barrel length.

I'm a little out of my depth here, but it would seem to me that double shots (two darts chambering from a hopper per shot) could be caused by too much airflow after the dart leaves the barrel. This may be fixed by lengthening the barrel.

I thought about double shots and didn't have a clear idea of the cause, but way too much air flow does seem highly plausible. Lengthening the barrel might help, but I'm not entirely sure as the only math I have for that is scaling. I'll have to think about it.

Also, I neglected to mention that my theory is based on the Venturi effect, which was previously hypothesized to explain how hoppers work. This idea combined with the high speed video in the thread were essential in figuring out what's going on. I did the math and found that the pressure drop from the Venturi effect is so small that darts can't advance unless the barrel gas flows into the surrounding air. This seems to fit with the high speed video. The darts don't advance until the current dart is out of the barrel and the barrel gas has had some time to drop to atmospheric.

I'll see if I have time to make a more detailed post about these findings this coming weekend.

Hmmm. I was writing a ballistics program to calculate ranges at different angles and such. If I had a decent value on the drag coefficient of various NERF darts I could make a few tables/graphs for reference.

Sounds nice. In the post directly prior to yours, I linked to an earlier blog post of mine where I summarized the available drag data: http://btrettel.nerf...com/archives/78

Based on tests done by Daniel Beaver and koree, I would suggest using a value of 0.67 for the drag coefficient. This value fits my flat-fire (small angle) approximate theory very well and is in concordance with wind tunnel tests done for circular cylinders with flat and smooth noses.

You can read more about the flat-fire theory in my old notes: http://trettel.org/p...stics-notes.pdf

This feature of NerfHaven is on indefinite hold. I do not have time to contribute.

I have not heard of internal fletching and can not find anything on the subject in a quick Google search. Let me know when you find something from the episode of the show you are talking about. I'll take a closer look if I have the time.

btrettel's Nerf blog was my blog about engineering Nerf guns. There I discussed various aspects of Nerf gun design with a particular focus on theoretical and experimental analysis. The goal was better Nerf guns.

Now it's a column titled Nerf Engineering in the articles section here at NerfHaven. The focus and goal are the same, but the audience is much larger. Despite that, I will make no attempt to dumb down the content. I write for someone in college or older. Some background in calculus will be required for some posts.

Expect a post every week or two. I have a large backlog of topics I've already analyzed, but have not written about in usable form. You can post requests here, in this thread, for now.

All posts here will potentially be updated after their original posting. The date of last update will be listed at the bottom of the post.

Are you going to be taking attendance, do we need to print out a copy of the syllabus, and when is the first exam?

I find this to be amusing.

Looking forward to the Flywheel lecture.

I'd like to second the request for a flywheel post. There's little to no knowledge in the community as to how variables like wheel mass, moment of inertia, and motor torque affect performance. If we could get a Real Life Physicist to teach us the basics it could really help people who want to build their own.

I know little to nothing about flywheels, but they certainly are within my skillset. I'll take a closer look. Feel free to post some more information here if there's anything in particular that you find interesting (like, for example, someone else's thoughts about flywheels) or other particular questions you might have about flywheels.

I would recommend lecturing about shady ass generic Chinese valves. They are fairly difficult to come across any factual information about them.

Valves in general have a lot of issues. I'll take a closer look at this. I do know the Asian valves tend to use a different test method than the American ones, which adds to the confusion when you do have some information.

If you haven't, calculate the approximate drag force of a micro dart in terms of velocity (in ft/s for use with chronographs) and mass (in grams).

This way, we can use the drag force and gravity to create another formula to get an approximate range from Chronograph readings. This would only work for accurate darts, though. So not for streamlines.

All this has basically been done, though it's not well organized. See this old blog post about drag for the drag coefficient of a Nerf dart and similar shapes and my old ballistics notes for some equations to estimate range. One of my plans is to take that earlier blog post and some appropriate sections of my notes and expand on them in a larger post. I'd like to make an online calculator for range, too.

Sorry about that, when topics get this long I tend to print them out and I did 3 days ago. Anyway, I am NOT assuming that you are shooting one large blast at once, the topic title with the words "semi-auto" explain that well enough. The CPS chamber actually will pressurize a smaller chamber like that very easily (that 9/16" chamber I described probably won't expand without having 80+ PSI on it). However, at this point you might just make a large air chamber and accept that there will be a performance drop-off because it will be easier to construct.

Anyhow, I myself had made a homemade that shoots a short burst of air with each shot. I was able to get 6 or 7 shots out with about 60-70 feet in range (the last shot had less because there was about half the amount of air as the other shots), all in about 40 pumps. The only thing that regulated that was the mechanism that opened and shut the valve.

I'd also like to know how this is going to be semi-auto, which to me is one trigger pull equals one dart shot, nothing special inbetween shots.

I think you have measurements wrong? I just measured the width of one side of an inner tube. 1 CM. 30x1cm=30 cm=over one inch. Now look at it because it is a circle, and it is almost 3 inches big, before even pumping. Look at a ruler. You'll be surprised at the size of 3 inches.

I believe that it was thickness I was measuring. Not the width of the tube. That depends on the ID of the tube, which doesn't matter much in this situation.

I tried to stay out of this, hoping that you'd eventually figure it out, but now it's just ignorant.

http://nerfhaven.com...?showtopic=2587
http://nerfhaven.com...?showtopic=2662

While you couldn't exactly find this topics using the search, they were recent enough for me to pull up without going crazy.

You can buy foam to make homemade darts. You can buy rubber tubing to make CPS bladders.

I'd suggest that all of you interested in this look at both of those topics. Also, read this PM I sent to someone.

http://nerfhaven.com/forums/index.php?showtopic=2587
http://nerfhaven.com...?showtopic=2662

Read the few topics on this and milk them for what they are worth. And if you've got anymore questions ask me or ask those on my forums. Actually, don't ask them, I really am the only one in the entier world who has used this stuff. I'm reuploading the images of my gun, which at this point needs work. The pump didn't last, needs a new one.

I get asked this a lot actually, but you're the first Nerfer to personally ask me. It doesn't matter what size you use. I am serious too. All the matters, power-wise, is total thickness. I will recommend that you buy the following however to make a good, powerful chamber. Be aware that these are two different sizes of LRT, one is to be put over the other and add resistance. This will produce a Super Soaker more powerful than the CPS 2000, please use caution. If you remember, a CPS 2500, weaker than the 2000, can shoot a Stefan over 250 feet.

ID: 3/8" Wall: 3/16"
ID: 1" Wall: 1/4"

Total thickness will be 7/16", and with some bike tubes you could get that up to like 9/16".

I only used about a foot of this stuff. I bought about 3, now I'm wishing I bought more. Buy a bunch of this stuff, if you get 25 feet or more of one type then it's cheaper. After that you can lie and say it's like PETG, only available in bulk and sell a ton. Or maybe I'm just giving you ideas.

If you look at my pictures, for this gun I closed the open end of the chamber, with a bolt and tubing clamps. That may not work very well on your very thick chamber. I would recommend getting a female threaded endcap, screwing in a tubing barb with teflon tape and then putting that on with a tubing clamp. DO NOT PUT IT ON TOO TIGHT! It will cut into the tubing, put some duct or electrical tape over where you are putting it if you want some protection from that.

Hopefully this helped.

Believe me, I know more than most anyone on this stuff. I'm as close to an expert as you'll get. Pressure does not matter, force does.

Think of this: My air pressure water gun has a pressure of about 100-110 PSI, and my CPS homemade has at most 60 PSI. The CPS one beats it in range by several feet. Why? Because pressure doesn't matter with CPS stuff. It's a well established fact at the water gun sites that CPS is lower pressure for equivilant power. Wasn't it Gaebo who modded a CPS 2500 and it shot like 250 feet? On the package, it lists the PSI as 22. The mighty CPS 2000's was listed as 26.

Plus, I don't think that the PSI listing is how much pressure is created. I think it's more like how much pressure is needed to make the tubing expand. It doesn't matter though, what ultimately matters most is total thickness.

Also, it will take 30+ bike tubes to make something as thick as what I said in that PM. And your RF20 bladder probably is a about as thick as the 3/16" thick tubes.

As I said (or meant), you all could learn a thing or two about CPS systems. The best way to get experience with the stuff is the buy it, so go and buy what I said to (buy it in black, I think the amber stuff might be different).

Edit:

If you want to get technical, technically CPS systems don't need to create any pressure at all. The collapsing of the tubing will force air out just like a chamber decompressing. No pressure is needed.

Done.

You might want to implement a way to note that the nerfer can be at two locations. A simple separator like | should suffice. PHP makes that easy with split(). I switch between Jefferson, MD and College Park, MD during the year so this would help people like me.

The ranges are completely true. I'd know because I'm his brother, and I helped him out with most everything (not too much design though). The valve opens fast, the pressure chamber is huge, the pressure is moderate, the barrel is several feet long: it's a proven formula for range. If we increased the pressure from the moderate 15 or so PSI we use right now, it'd just be plain dangerous (or more than it already is). Already at barely 15 PSI we've had accidents involving glass...

The 250 foot shot is at least short. We never recovered the dart simply because it shot into a neighbor's yard, right over their house (it was an angled shot). We measured from inside our garage to their house and took that measurement (with a tape measure). The gun is perfectly capable of more with more pressure, but I wouldn't want to be near it.

Dr. Nerf and I don't use normal "stefans". Dr. Nerf had the idea to use small, relatively heavy screws in Nerf darts because we don't have to use hot glue given you can screw them into the foam. This is easier to make, cheaper and faster. I'd also like to say they perform better, but I bet if anything it's simply added the weight that helps.

What kind of spring did you use for the magazine? Just curious because I know how hard it is to come up with a spring good enough to use for a magazine.

To the best of my knowledge, he used multiple springs. I bet you'd rather hear it from the horse's mouth though, as I'm not completely sure.

To clear up some confusion, this system can have potentially infinite shots. Right now he has the gun setup to be attached to the air compressor in our garage just like many air powered tools. I plan on letting him use my Constant air pressure water gun's air chambers after I build it - this should provide about 22 liters of compressed air stored at at least 100 PSI. I'd have to do some math to figure out how many shots that would be, but you get the picture.

I am glad that they finally let Andrew in here though... this was his third attempt at registration.

Quote: Dr. Nerf
And second, 150" is a far distance away.

Woah, I never knew one hundred fifty inches is a far distance. O_o

This is one thing I've learned to hate about this forum: the constant ass kicking and sarcasm. Stop it. You could have said that nicer. I do like that you chose a name that fits your personality though.

Could you say a little bit more about how you made the PVC pump you mentioned? Also, supposing that a pump on an existing nerf gun (say an AirTech 3000) broke, would it be possible to use a PVC pump like the one you made as a replacement?

I'm not him, but I'll tell you that the pump he made was really simple. It only has one check valve, so you have to remove the piston each time to replenish the air. If you want to replace the pump of another Nerf gun with a homemade one, that's not a problem at all. Visit my website and look at the "Air Pressure Homemade" article for more information on homemade pumps. If you still have the piston from that pump that would be better, because it can be hard to find O-rings that seal and move well in sch. 40 PVC pipe.

Once you load the bolts into the clip, what keeps them from just flying out?

I believe he loads the darts into the part where the clip is held, and then pushes the clip over that. You're completely correct - they would fly out of the clip.

Use a lathe when you need a centered hole. Don't most people put their darts in a lathe to get a perfectly centered hole? I do.

I'm a maybe for the moment. Whether or not I can attend depends primarily on whether I can get a ride. My work schedule might be an issue too, but I doubt it will be.

Great work as usual. I encountered the earlier prototype as well and heard a bit about the project goals, and I was impressed then as I am now.

CS, you mention that you haven't found any "ring shaft seals" that fit this tubing. I was actually looking to use the same size tubing with piston cup seals (9411K13 on McMaster-Carr, specifically). As far as I can tell, piston cup seals should work well for a 2 inch piston. If you haven't already tried these seals, I would suggest them.

Yeah I ordered one of those last year to inspect it, they won't work.
Those are meant to seal at thousands of psi inside of hydraulic rams. They're made of a composite fiberglass and are extremely hard.

Any particular reason you don't think they'll work? I've used them for years in water guns. See this water gun for an example.

At first I thought they wouldn't fit in the pipe, but once I hammered it in and lubricated the seal, I was very happy. The friction might be a little higher than usual, but the seal was perfect. And they're very straightforward to install.

Here's the tl;dr version:

• Front-weight darts to make them stable.
• Make darts consistently. Keep the shape consistent. Keep the mass consistent (and measure it to check).
• Make darts well.
• Use systems that remove human variation from the gun, like piloted valve systems (i.e. "backpressure tanks").
• Increase the dart mass.
• Increase the muzzle velocity.
• Use a stiff barrel.
• Use a sight.
• Use the barrel length that maximizes performance.

These changes are very likely to improve accuracy and precision. The remainder of this thread is details about why these changes will probably improve accuracy and precision and some requests for testing to see what works best and what doesn't work.

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In light of some recent discussion of rifling as a potential cure for inaccuracy and imprecision of Nerf darts, I decided that we should discuss how to actually improve accuracy and precision. It's a shame that so much effort has been focused into rifling, which probably can't work. Let's focus on what stands a good chance of working.

(I have already explained why rifling most Nerf darts probably can't have any significant good effect and how tests so far have only shown that rifling doesn't seem to improve precision or range.)

Wikipedia has a good page about the difference between accuracy and precision. Basically, accuracy is how close the darts are to their intended target and precision is the spread around where the darts hit.

What is most central to accuracy and precision is repeatability. We want the trajectories to be as consistent as possible between shots. Consequently, we'll be looking to eliminate sources of variation.

Dart stability seems to be necessary for high accuracy and high precision. Stability here refers to resistance against overturning or perhaps even fishtailing. If a dart overturns of fishtails significantly, it is subject to aerodynamic forces that are inconsistent from shot to shot, so the accuracy and precision are affected greatly. The range is generally also very significantly affected.

We know that front-weighted darts have their center of gravity far in front of the center of pressure (where the net aerodynamic forces act), and this leads to stable darts. The darts rotate about their center of gravity, so when the center of gravity is basically at the tip of the dart, the dart basically can only move the length of the dart into the air stream. This applies a force that will rotate the dart back until it is parallel to the flow. This is why front-weighted darts are stable. If they started to become unstable, they'd correct themselves.

There are many other sources of variation that are worth looking in to. We want to ensure that darts are consistent. A few things to keep consistent: dart masses, weight distributions, and drag characteristics. The weight distribution can be kept consistent by using the same materials for all darts and carefully positioning and sizing all of the dart materials. The drag characteristics basically are shape of the dart; we don't want some darts to be bent, have different tips, have hot glue sticking off one end for some darts, etc. Here's the short version: making better and more consistent darts will help accuracy and precision.

Another source of variation is muzzle velocity. If your gun shoots strong on one shot and weak on the next, then obviously where you aimed for the first shot is probably not where your dart will land for the second. Muzzle velocity can be made more consistent by identifying sources of variation in the gun-dart system. Dart mass must be kept consistent here again. If a valve is used, it's better to use a piloted valve as they open basically instantaneously and the time to open is relatively independent of how quickly the user pulls the trigger. If a spring gun is being engineered, try to design the system to make the catch release at the same rate regardless of how quickly the user pulls the trigger. I could go on; you get the idea.

The actual dart mass is also important in addition to its consistency. Heavier darts are more resistant to wind and other transient forces that will push your darts off course. This, unfortunately, conflicts with some people's safety guidelines, but perhaps those guidelines can change in the future.

The actual muzzle velocity is also important for similar reasons. Higher muzzle velocities can lead to lower transit time (the time from leaving the barrel until hitting the target) for the dart, which should reduce the total impulse applied to the dart from transient forces like wind. And as we know, lower impulses mean lower changes in momentum, which means the dart will be more on target. Of course, the muzzle velocities could increase so much that drag causes the transit time to increase. I think this is an area where we need to do some testing before figuring out what works well. Increasing muzzle velocity could potentially counteract some of the limitations in accuracy and precision seen with dart mass restrictions.

The Spud Wiki's page on spudgun accuracy notes that barrel vibrations can cause inaccuracies. I recall this being mentioned on some real gun ballistics websites too. I'm not too sure how significant this effect is for Nerf, but it might be more pronounced for more flexible barrel materials like PETG. A few tests to see if the barrel stiffness (the product of the modulus of elasticity and the second moment of area) has an effect on accuracy seem to be justified. I have a table listing the stiffnesses of some common barrel materials.

A number of other things on the gun can be changed to improve accuracy and precision. Sights are an obvious addition. Simple iron sights are plenty adequate for most Nerf guns, though, I am intrigued by the possibility of using ladder sights with angled shots, but that's for another post. The sight radius, the distance between the front and back sights, can also be increased to allow you to be more precise when aiming. Longer sight radii make aiming errors more noticeable.

These are the main things I'm considering when looking to improve accuracy and precision. Does anyone have any other ideas or thoughts?

I think we'll have to do a lot of testing to figure out some other things. I'm curious about the relationship between precision, dart mass, and muzzle velocity. I had planned to do some tests to determine this relationship last summer but I never got around to it.

I can think of one reason why more aerodynamic darts might be more accuracy or precise. More aerodynamic darts can maintain their speed better than other darts, so their transit time is reduced, which should reduce the total perturbative impulse (as previously mentioned) from wind, etc., that will push them off course.

I'd be interested in whatever tests you do. We all should do more tests and share the results.

Extra air shot out of the barrel when the dart leaves the barrel is called muzzle blast. This could have a negative effect on accuracy and precision because it's difficult to predict. Porting is one way to avoid this problem. Another way is to use barrels that are just long enough for the dart to leave without excess gas escaping. Thankfully, this coincides with the barrel length that maximizes performance (for low-speed guns, at least), so using the optimal barrel length can improve accuracy and precision.

Barrel length brings to mind another item in the myth category along with rifling for Nerf. Some people seem to think that longer barrels somehow make things more accurate or precise. This is probably true to some degree, that is, barrels that are shorter than the dart probably aren't very accurate. Some people seem to think that constraining the darts to move in a straight line helps somehow. But this is based on a poor understanding of Newtonian mechanics. When the dart leaves, it's a ballistic projectile. Where it was in the past is irrelevant; the conditions when it leaves are all that matters.

Once the barrel is long enough, then its length does not influence accuracy or precision unless increases in length change the sight radius, vibration characteristics (i.e. natural frequency) of the barrel, muzzle blast, etc. And it's obvious that longer is not always better here.

I've been busy, so there's a lot to reply to.

1) I seem to recall a thread (by you, perhaps?) to determine optimal barrel lengths for airguns.

2) Also, what about 'telescoping barrels', such as 6" of CPVC at the breech end, hammered into 12" of PVC?

1) Yes, I posted a thread about mathematically determining optimal barrel lengths. I've been tinkering with different analytical models in my (nearly nonexistent) free time, so I'll probably post an update about all this eventually.

2) This sounds like a good idea to test. It'd hard to predict how well it might work.

Ooh! Porting a barrel may be an easy, effective way check your barrel length!

This is a pretty creative idea. Good thinking. This sounds like it should work, but I personally would find it to be a hassle. If you want something cheaper than a chronograph to do measure optimal barrel length, I would suggest a ballistic pendulum.

For consideration as you didn't really touch on this:

http://youtu.be/dITQvgxZS7k

The barrel wobbles in the coupler, this probably isn't a problem for most but it does happen.

That's a very interesting video. Thanks for posting it. I see more than wobble in the coupler too. We should test springer vs. air gun accuracy and precision (for about the same muzzle velocity, dart mass, etc.) to see if these effects are significant. We should also test couplered barrels vs. firmly fixed barrels.

It's also worth noting that it seems that only the first vibration mode is excited. This simplifies modeling of the barrel's vibrations, if we want to do that to get a rough idea of what we have to gain by using different barrel materials or configurations.

1) When you have an overly short barrel (even by a matter of a few cm) as the dart exits the barrel excess gasses will continue to accelerate the end of the dart resulting in fishtailing or tumbling. To see this in real time it is easier to use a longer dart. This is because fishtailing is easier to identify. This effect is generally not as profound with short stephans (because there is little distance for the back of the dart to be accelerated.

2) Oops...I just realized I basically repeated Doom's second post, except he did it much more succinctly and effectively... Well, at least the whole thing about the high speed camera supports Doom's knowledge...

1) It's nice to have experimental verification of this. The bit about short darts is interesting. I'd be interested in seeing the footage like shmmee, if that is possible.

2) What you've posted is definitely appreciated. Explaining what I said in a different way is good. Also, I don't know very much. I just think about what little I do know. A little knowledge goes a long way.

Has anyone produced a rifled barrel?
Spudfiles has a how-to rifle pvc that I found interesting.

I linked to a thread here where someone rifled a barrel. Look at the first sentence past the divider of the first post.

The other thread I linked to has some tests too. The link "Rifling debunked" in my signature also has some tests.

I did substantial trials with the Quadshot because as I said before

It doesn't seem that you did any statistical analysis, so just testing alone is not enough.

If you recorded the data, I would suggest posting it so that we all can analyze it. As I've said, just observing a difference in the average (for example) is not enough to say there is a difference. You have to use statistical tools to analyze the data to see what's going on.

I know that I don't have many posts here on Haven but that doesn't mean that I'm a noob. I'm not trying to pick a fight or anything Doom, I just wanted to clear that up since I haven't been very active on the boards and I know that it could be a concern.

I didn't call you a noob, so please don't imply that I had or that I might think you are one. My posts might come across as combative, but I assure you that this is not intentional. The point is to have good data supporting a hypothesis.

Your data probably is good as the idea behind porting is based on sound science, however, let's be sure.

Thanks for doing some tests Irish8.

Not to be a buzzkill, but how many trials did you have?

To be clear (as I was in previous rifling threads) just doing a few shots and seeing a difference in the average is not enough to show there is a difference. You need to use statistics to show the difference is statistically significant, that is, more than what you might expect from the variation seen normally.

The barrel is *not* detachable. It's fixed. Hasbro actually spent a hell of a lot of money with a ballistics University in the US, who proved barrels benefit blasters, and rifling actually does also. So it's something Hasbro are sticking with. They, at my last conversation with Engineers had no intention to make a detachable blaster.

Do you have any more information about these tests? Which university did they fund? What do you mean when you say that "barrels benefit blasters"? (Of course they do. Try a gun without one.)

Is this gun rifled?