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Establishing barrel length

Stupid simple

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#1 shmmee

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Posted 23 May 2011 - 11:26 AM

I'm not taking pictures of this write up, because if you're so dense that you need a picture of a dart stuck in a barrel to grasp this concept, you should be playing with playschool toys, instead of hasbro.

I've been thinking about simple ways to establish a proper barrel length, and after trying to use a big salvo to fire a steffan out of 1' of pteg, (dart didn't leave the barrel) this thought kinda walked up and slapped me.

Start with a long barrel - too long for the dart to leave the barrel, and fire an unweighted fbr blank. Because the steffan is unweighted, it will have little mass, and very little momentum. It will also stop suddenly as soon as it begins to pull a vacuum. Pull off the barrel, use a rod to measure how far the dart made it down the barrel. Repeat to find the minimum length traveled.

I haven't done any testing to see where the best barrel length is in relation to where the fbr blank stops, but I would guess it wouldn't be more than 1" behind the minimum length traveled.

Approaching it from this method should spare you walking down ranges, or measuring muzzle velocity.
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#2 Daniel Beaver

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Posted 23 May 2011 - 11:53 AM

Interesting approach. But...

I haven't done any testing to see where the best barrel length is in relation to where the fbr blank stops, but I would guess it wouldn't be more than 1" behind the minimum length traveled.

Most springers we use have plunger_volume/barrel_volume ratio more or less in the 4/1 range. The fbr does not stop because the vacuum equalizes, but rather because the kinetic friction of the barrel and the force the pressure difference balances out. For airguns (where barrel friction is ostensibly zero), I think you will find that this method will yield obscenely long "ideal" barrel.

This might be a useful method of finding the point where parasitic forces begin to cancel out the positive forces driving the dart down the barrel. I need to do some pencil-and-paper, but I have a hunch that there is a flaw in the method you are proposing (not sure what it is, just a hunch).

Edited by Daniel Beaver, 23 May 2011 - 11:55 AM.

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#3 shmmee

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Posted 23 May 2011 - 05:01 PM

Ok, I'll trust you guys on it. I failed to take into account different darts. I guess it was one of those things that sounded simple and perfect in my head. Sorry about accidentally posting a concept instead of something more proven.

In an attempt to salvage this into something bennificial, is there any easier method than taking muzzle velocities, or measuring ranges (both of which would require many, many test shots) when establishing a barrel length?
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#4 Daniel Beaver

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Posted 23 May 2011 - 10:52 PM

In an attempt to salvage this into something bennificial, is there any easier method than taking muzzle velocities, or measuring ranges (both of which would require many, many test shots) when establishing a barrel length?

Not really. Which sucks.
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#5 Langley

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Posted 23 May 2011 - 11:09 PM

Ok, I'll trust you guys on it. I failed to take into account different darts. I guess it was one of those things that sounded simple and perfect in my head. Sorry about accidentally posting a concept instead of something more proven.


Lots of people don't really agonize over their length as much as you would think from reading this thread. But Bob's inadequacy issues aside, this method seems like a perfectly good rule of thumb. Much better than my strategy of using whatever size piece of PETG I have left over.
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#6 TantumBull

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Posted 24 May 2011 - 01:33 AM

One other thing to consider is that a dart with less momentum, in addition to being able to stop sooner, will also begin to accelerate sooner. Just some food for thought, I have no idea how much it would matter.
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#7 Doom

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Posted 24 May 2011 - 06:36 AM

Interesting approach shmmee. Unfortunately, it can be hard to know when this is applicable. I'll offer a more significant problem with using light darts than what has been mentioned. Optimal barrel length is a function of dart mass to a certain extent based on my experience and my computer simulations. I've noted before that when the darts are heavy enough the ideal barrel length is independent of dart mass (as long as this criteria is met, at least). But when the darts are extremely light, it's likely that the pressure in the barrel is going to be different than the pressure in the gas chamber or spring chamber, and this means that your optimal barrel length will likely be significantly different from what it would be if you used "heavy" darts. Is is that bad? I guess it depends, and testing would be the easiest way to see if it is bad.

All the rambling about friction also applies, but based on my simulations, it's not as significant as people think it is. And it's certainly not as significant as the effects from using a low mass dart.

To me, the easiest thing is to have a few barrels of different lengths and a chronometer. I don't really know what anyone's trying to avoid here as barely anyone has even bothered to use a chronometer. It seems to be common knowledge that this sort of testing is bad, but it's not that bad. People seem to spend a lot more time trying to figure out how to avoid using a chrono than it would take to use a chrono. I've done tests like this and it's not that hard or even that annoying. I spent perhaps an hour or two to measure 75ish shots.
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#8 shmmee

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Posted 24 May 2011 - 08:27 AM

Interesting approach shmmee. Unfortunately, it can be hard to know when this is applicable. I'll offer a more significant problem with using light darts than what has been mentioned. Optimal barrel length is a function of dart mass to a certain extent based on my experience and my computer simulations. I've noted before that when the darts are heavy enough the ideal barrel length is independent of dart mass (as long as this criteria is met, at least). But when the darts are extremely light, it's likely that the pressure in the barrel is going to be different than the pressure in the gas chamber or spring chamber, and this means that your optimal barrel length will likely be significantly different from what it would be if you used "heavy" darts. Is is that bad? I guess it depends, and testing would be the easiest way to see if it is bad.

All the rambling about friction also applies, but based on my simulations, it's not as significant as people think it is. And it's certainly not as significant as the effects from using a low mass dart.

To me, the easiest thing is to have a few barrels of different lengths and a chronometer. I don't really know what anyone's trying to avoid here as barely anyone has even bothered to use a chronometer. It seems to be common knowledge that this sort of testing is bad, but it's not that bad. People seem to spend a lot more time trying to figure out how to avoid using a chrono than it would take to use a chrono. I've done tests like this and it's not that hard or even that annoying. I spent perhaps an hour or two to measure 75ish shots.

I completely agree that a chronometer would serve as the ideal method, I've priced them before and have decided they are beyond my nerfing budget (though I do have a bone stock cross bow I'd be willing to trade for a decent chrono - if any one is interested Yeah, scratch that offer. 3 hours sleep can make you say stupid things). I was just trying to come up with a simple - low budget method for the many who are in the same boat as me.

I might look into the theory some day later, but I'm worried that it might end up being as much of a boondoggle as the rifled barrel discussion (which I foolishly let myself get sucked into). Had I the time of an uncommitted teenager, I would be more than happy to examine this concept at length, but screw that. I've got kids. I feel lucky to have an undisturbed 3 hours to mod, and defending this concept lays so far at the bottom of my list of nerfing priorities, it's picking pre-chewed gum off of it's face. Thanks for the input everyone,

Doom - as usual - you rock.

Edited by shmmee, 24 May 2011 - 12:10 PM.

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#9 taerKitty

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Posted 24 May 2011 - 10:26 AM

Doom - as usual - you rock.


I'm willing to risk a verbal bitch-slap from an admin or mod for contributing nothing to this thread just to say:

Ditto, Doom. You fscking rock.
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#10 MrPzowned

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Posted 24 May 2011 - 03:07 PM

"Experimentally the plunger volume seems to be about 4 times that of the barrel. The relation for barrel to plunger size can be summed up in the following equation,Posted Image
where rb is the barrel radius, rp is the plunger radius, lb is the barrel length, and lp is the plunger length. For Nerf applications the barrel is almost always 1/2" PVC or CPVC. rb can then be set as a constant at 0.25" and removed from the equation. Since we are trying to solve for the barrel length with a given plunger size, the equation can be rearranged and simplified to:Posted Image
This simple equation makes it easy to roughly but quickly size a barrel to a given plunger. The equation could also be used to size a plunger for a given length barrel. This equation is based on experimental data and is not perfect. Four is not the golden number. This produces the optimal barrel length for the situation I was testing. The type of dart, dart-barrel friction, and total system volume will likely effect the optimal ratio. Nevertheless, the above equation can be used as a starting point." - Boltsniper.com
Found this while looking at the FAR. If the photos have trouble showing up go to boltsniper.com and it is under the FAR section in homemades.
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#11 Possemhunter

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Posted 24 May 2011 - 03:29 PM

If you use an airgun and are one of those people who likes to hide in bushes, taking pot shots, spray a load of wd40 in the barrel before shooting. Watch how far it goes.
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#12 Doom

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Posted 24 May 2011 - 05:55 PM

I was just trying to come up with a simple - low budget method for the many who are in the same boat as me.


I can understand this. I just bit the bullet and bought a cheap chrono. I'd be happy to lend it to you if you were local, but unfortunately you're across the country.

There are homemade chronos (basically microphones with the right spacing) and ballistic pendulums. Both are cheap ways to measure muzzle velocity and probably are fun projects by themselves. As I recall, SpudFiles has a lot of info about both.

In reference to bolt's work, some people in the past seem to take that equation as holy scripture spoken by a Nerf deity, despite bolt's stated conditions for when the equation applied. He said this equation "produces the optimal barrel length for the situation I was testing." That's it. It doesn't apply to other situations. And I would actually challenge those who claim this offers a barrel length that maximizes muzzle velocity.

Bolt said his "goal was to find the barrel length for which the dart would exit the barrel as the plunger reaches the end of the plunger tube". However, we have no reason to think that this time coincides with the time that maximizes the muzzle velocity. In fact, based on my understanding of interior ballistics, I would expect the muzzle velocity to be maximized if the dart leaves the barrel after (not when) the plunger hits the end because the gases will expand more.

I posted some equations for theoretical ideal barrel length a while back. These equations can be derived from first principals with some simplifying assumptions and are almost certainly correct when those assumptions are valid. Should you regard these as perfect? No, as they really only apply for very heavy darts.
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#13 shardbearer

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Posted 28 May 2011 - 01:22 PM

Technician over at spudfiles came up with an interesting way to find the optimal barrel length. He wrapped coils of wire around his barrel at regular intervals, and connected them to the microphone port on his laptop. He embedded a neodymium magnet inside his projectile (a tee shirt), and recorded the "sound" of the magnet going through his barrel. Timing how long it takes it to pass between each coil and the next, you can figure out the velocity at any segment of the barrel, and therefore when the magnet stopped accelerating, which is the optimal barrel length. He detailed his process here.
The easiest way to put a magnet in your dart would be to replace the washer in a slug dart with a disc magnet, but disc magnets small enough to not throw off the weight are hard to find unless you order online.
The best magnet I could find was this one, 7/16 in diameter and 1/32 thick, weighing in at .577g, very close to the .64 and .52 of #8 and 6 washers. It is N52 Neodymium, the purest variety, and cheap at $.50+shipping.

Edited by shardbearer, 11 June 2011 - 01:55 AM.

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