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Pulley system Bullpup style surgical tubing powered blaster

(With new style hopper). The Dwarf. It feels good to be back.

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#1 Boot

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Posted 01 July 2012 - 03:53 AM

After a long absence from nerf (or at least building nerf blasters) I finally graduated from the IB, and have had some time to work on the nerf ideas I have accumulated over the past year or two.

As with my other posts, I tend to ramble on with somewhat unnecessary back stories, so skip ahead if you are just here for the pretty pictures.

The Dwarf is the result of my attempt to develop a blaster with one simple goal in mind: minimize blaster size without impacting range or rate of fire. i.e. create a relatively small and maneuverable blaster that can compete with more well established homemades like the PACs and pumpsnaps in terms of both range and rate of fire.

To accomplish this goal I split the standard spring powered nerf blaster into what I consider to be its three main components, the plunger tube, plunger rod, and barrel. By folding the three of these over each other the resulting blaster is about a third of the size of its standard equivalent in terms of range. To achieve this I replaced the standard rigid plunger rod with a high grade flexible steel wire, allowing the plunger to be reversed without reversing priming direction. This resulted in a 22" long blaster even when primed and with a barrel with a ridiculous 8" draw.

While many don't like the pulley system due to it's perceived unreliability and complexity a few simple adjustments make the system just as durable and reliable as any other spring powered blaster system. The three primary changes are the use of a large diameter pulley instead of a smooth bolt, the use of a slightly more rigid plunger cable, and the use of an extension style powerhouse, in this case surgical tubing.

At this point I think it is important to give credit to both Carbon and Kane the Mediocre, without which this blaster would not exist. Kane was the first one I saw use a pulley system for a plunger rod, giving me enough confidence in the design to move forward with the project, and carbons introduction of both the Snap catch and extension spring style blasters provided me with simple solutions to otherwise complex problems (i.e. figuring out a functional catch, and more reliable powerhouse)


SKIP TO HERE FOR PRETTY PICTURES.

Without further ado I present to you "The Dwarf", small, but still packing a major punch.

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Exploded (note that disassembly to this degree only requires the removal of a single bolt, allowing for easy repairs of the plunger head).

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You may notice the build quality is significantly better than my previous projects. I spent much more time and effort on this build, and tried to create a functional end product, rather than just another proof of concept prototype that I usually produce. The end result is a sturdy blaster which is one of the most effective "spring powered" blasters I have ever owned.

Operation of the blaster is simple. To minimize size, while still allowing the blaster to be shouldered I opted to use of "bolt action" type priming mechanism, with the priming bar on the side of the blaster. While this is slightly less intuitive to use than normal pump action setups, after a while of use it becomes equally effective. This priming system allowed the grip to be at the very front of the blaster, balancing it out better, and cutting off a few inches in length since the priming handle and plunger tube are in parallel.

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The build is roughly similar to a Snap, operating off the same style of trigger, and utilizing the same types of materials, but that is where the similarities end. As is always the case, the materials available to me here in China are very different from those in the US, so I will simply explain the operation and construction of each part without providing concrete measurements (I will also avoid providing measurements because I am lazy, and more or less just eyeballed everything anyway).

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The plunger head is fabricated around a single bolt for strength, and is composed of three parts, each of which is made of a separate PPR endcap. The catch-face is also where the steel cable anchors by means of two holes which it is looped through, and then fastened with a wire clamp (left of image). The plunger itself is a cup seal from a ball pump (center). To ensure a perfect seal the PVC body of the blaster is not actually the plunger tube, just a shell. A large diameter ball pump tube is nested in the PVC. The slight gap between the pump tube and the shell is what allows the mechanical fastening of the grip and priming handle guide rail.

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Finally, in front of the plunger itself is a pvc buffer to protect the cup seal. This is also what the surgical tubing anchors to (explained later).

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One issue people have previously had with this pulley system style of blaster is the friction from pulling the "plunger wire" around a 180 degree bend. I have not however found this to be an issue. When developing this blaster I created a similar build with a traditional plunger setup with the same draw and power system (pictured below) to experiment with the most effective setup for using surgical tubing to power a blaster. The build (see below) was very similar to Carbons Snap Revolution (with the exception of the priming system), and performed almost identically to The Dwarf. I assume this is because the reduction in the mass of the plunger rod saves an equivalent amount of energy to the friction generated by bending the wire.

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One of the main innovations in this build is the use of a large diameter pulley. The pulley itself is not some specialized component that you will never be able to find. It is just a replacement wheel for an office filing cabinet with a channel dremelled in it. The channel acts as a guide to keep the plunger wire in place. If you do try to replicate this build (which I sincerely hope some of you do, just so you can experience to awesomeness first hand) make sure to bevel the edges of this channel. If this is done properly even when the wire "derails" priming the blaster again will seat it back in place.

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One VERY important component is the endcap at the front of the blaster. Drilling a small centered hole for the wire to slide through keeps the plunger wire centered on the pulley. Without this component the wire will quickly slip out of position and jam the blaster. This component also helps keep the blaster primed by keeping the plunger head aligned, and keeping it from slipping it off the clothespin catch.

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The primary advantages of using a large diameter pulley are threefold. 1, since the pulley itself is larger is will rotate slower, reducing rolling friction. 2, with a correctly sized pulley the plunger wire in the plunger tube, and the wire outside in the priming system are more or less in parallel, meaning there are no angular changes in the wire as the blaster is fired or primed. This results in a significantly smoother priming motion since the vector of the force opposing the priming motion remains unchanged throughout the entire pull (i.e. no extra friction by pulling the priming handle sideways agains the guide rail). 3, especially when using steel cable, a small bend greatly increases friction, as it bends the wire past its elastic limit. A larger pulley keeps the wire under its elastic limit, allowing for a smoother drawing and firing motion.

Also note the use of the 45 degree L to mount the pulley. This allows the pulley to be mounted more precisely since its placement can be controlled in two dimensions (sideways and forwards/backwards), which in turn helps reduce friction as described above.

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The blasters power source is also fairly unique. It is similar to Carbons Snap Revolution design, in that it pulls rather than pushed the plunger. As such it possesses the same benefits:

Greatly reduced plunger friction means a weaker spring can be used. Easier to prime, equal performance.
Safety: when the system is primed, the spring is trying to pull the blaster together, rather than blow it apart.
rod
Having the spring inside the plunger tube means no more snagging the nail on the spring.


The system is not however completely identical, namely due to the complete lack of a solid plunger rod in which to mount to spring. Instead the surgical tubing is mounted in front of the plunger head (see below), pulling the entire assembly (like in the "Ultimator" rocket launcher). While this does introduce a certain amount of dead-space, if you are worried about ranges one of the great things about working with surgical tubing is that you can simply increase the draw almost indefinitely. I currently have the blaster set up with a ridiculous 8" draw (which puts it on par with a traditional blaster with 6-7" of draw), but there is no reason you couldn't do one with more than a foot of draw if you wanted to be ridiculous (and even then the end product would probably be smaller than a +bow with its barrel on). One of the great things about working with surgical tubing as a power source is that it is MUCH more flexible than springs, both extension or otherwise. You can vary tension, draw length, and draw weight by simply using more or less tubing, and wrapping it around a different number of times.

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The system releases air vertically, while housing the surgical tubing past the outlet as illustrated below:

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It is very important to support the assembly that houses the tubing because it is under a lot of stress when primed. The stock of the blaster cups the assembly, keeping the surgical tubing from bending the assembly sideways and breaking the seal on the front of the plunger tube.

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The priming system of the blaster is also fairly unique, and to the best of my knowledge hasn't been done before. A length of PVC, the priming guide, is bolted to the side of the blaster and a PVC T slides over it.

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Note that the T is cut so that it sits against the side of the blaster, and can snap on and off for easy disassembly.

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Otherwise this system is identical of standard pump action setups. The handle slides over the guide, and a bolt goes through channels in the guide tube. Inside the plunger wire is fed through a smaller piece of PVC that telescopes inside the priming guide and anchored on the other end.

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When the handle is pulled back so is the wire.

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Then the handle is returned to its starting position and is ready to fire.

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The feeding system of the blaster is also hopefully a new idea. I may start a separate topic just for it if people show interest, and if it actually hasn't been done before. It is a simple "wye-less" variant of the hopper, however several small modifications allow it to reliably shoot pretty much any "nano" sized foam ammo including streamlines, gumdrop darts, slugs, dome darts, etc. It combines the reliability of the RSCB with the rate of fire of the hopper. In actuality it isn't so much a hopper as an "SRSCB", a skewed RSCB.

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Its construction is almost identical to that of the RSCB with one extra step. Using this tool:

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You bend the barrel 45 degrees, and then mount the thing at an angle. Note that I am not sure if this will work with US materials like CPVC. My go to barrel material is the metric electrical PVC that they sell here which is much thinner, softer, and easier to bend than the CPVC that I have seen.

I have fired hundreds of streamlines through this SRSCB and have not had a single misfire (except one due to the wire derailing on the blaster, which wasn't the fault of the feeding mechanism).

The reason for this systems reliability is simple (Warning: rambling mode activated), and is best described by going through how I designed the SRSCB.

As you may notice from all my previous builds, I have always avoided using hoppers both due to the difficulty of getting wyes in China, and their inability to fire stock ammo (which is what we generally use here both because buying them in bulk from the factories is actually cheaper and quicker than making darts, and because it is easier to get new people interested when they already have some stuff to get started with). Instead I always opted to use an RSCB due to its reliability and ability to feed stock darts. The issues affiliated with feeding a streamline through a hopper were twofold. 1, their soft rubber tip made it difficult to get around corners (which some got around by sticking bits of cloth in their hoppers), and 2, their length and long rubber stems made bending them around a sharp corner difficult.

To fix these two problems I thought about how to seat the darts in the barrel before bending them around a corner, and about how to smooth out the bend in a hopper. By simply bending the barrel, and introducing the air source behind the bend a critical difference is made. Because the darts are already seated in the barrel when the blaster is fired air can ONLY escape if the streamline goes through the bend (if everything else is well made and airtight). As such unlike a conventional hopper which uses the preliminary burst of air from a blaster to blow the dart into position the full force of the blaster goes into bending the dart around the corner. This greatly increases reliability, reducing misfires. As a test of this system I actually created a 6" dart, just for giggles. It fed perfectly and went a good 90'.

The SRSCB is also more pneumatically efficient than a conventional hopper in that no pressure can escape from the system until the dart leaves the barrel. While my experience with normal hoppers is limited I believe that air can initially escape around the dart in a conventional hopper, until that initial burst of air properly seats the dart (please correct me if I am wrong). While this "waste" is negligible in slow air release spring powered blasters to my understanding this is why hoppers don't work as well on quick air release air tank/ air pressure powered blasters (since a greater percentage of the total volume of air released by the tank passes the dart before it is seated).

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As a result, while a conventional hopper may still be more efficient on a spring powered blaster (since the air doesn't have to go around a bend like it does in the SRSCB) the SRSCB has the added benefit of functioning just as well as an RSCB on air pressure powered blasters (which adds a whole new dimension to my old Mirage series blasters *wink*). The SRSCB also has the added slightly less significant benefit of not requiring a wye which I'm sure people in positions like me would appreciate (people out of reach from McMaster and other specialized hardware sites).

I know people will be asking about performance so:

General stats (with a foot-long SRSCB, powered by four strands of surgical tubing, 8" of draw)

Capacity: 6 streamline darts (or 18 1" slugs) but this can be pretty much anything you want...
Range: about 110-120', but again, this is variable. Four strands of surgical tubing offers a very smooth prime, increasing the draw weight and draw length would be easy.
ROF: How fast are your hands?

Also, remember to listen to Homer and drill "Speed Holes" (you don't have to use a pickaxe). On the first test I was getting like 40' ranges, and was really disappointed. Although they may not work on your car these holes did triple my ranges.

Of course I'll let you see all of this yourselves in the VIDEOS:

Blaster overview - shows how the pulley works, and how smooth the priming motion is.

Firing test - pew pew pew. Also, rockets.

Firing test of the test rig - for those who are interested.

All in all I am very happy with how this build turned out. With the introduction of only a few non standard materials (the filing cabinet wheel, cable, wire clamp, and surgical tubing), all of which should be widely available this blaster cuts standard pump action systems down by a factor of 3 in size while otherwise maintaining functionality. I am even more happy with the feeding system. One small change to the standard hopper has allowed for the use of stock streamlines in hoppers and, at least in my opinion, greatly increased reliability with only a very minor cost in range.

I am sure I haven't covered everything in this writeup, and am happy to discuss the pros and cons of pulley system, and extension powered blasters with anyone. Questions and comments both regarding the blaster and the writeup are very welcome. If anything was unclear don't hesitate to ask for clarification, I'll try my best to make myself clear.

Also, @ Taerkitty: I hope this changes your view of pulley system bullpup blasters :lol:

Edited by Boot, 01 July 2012 - 03:56 AM.

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#2 makeitgo

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Posted 01 July 2012 - 09:36 AM

Nice job.

Does the pully help decrease the draw strength needed otherwise by a 'straight pull?

What happens to the 'slack' of the cable when it's primed?
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#3 koree

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Posted 01 July 2012 - 10:35 AM

This is really cool! I especially like your use of the surgical tubing and the use of the 45 degree fitting to house the pulley in the front.

Does the pully help decrease the draw strength needed otherwise by a 'straight pull?


A pulley system like this doesn't provide an mechanical advantage. It only changes the direction in which the blaster is primed.

This should explain how and when pulleys provide a mechanical advantage.


Edited by koree, 01 July 2012 - 10:36 AM.

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#4 Carbon

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Posted 01 July 2012 - 12:58 PM

Oh, I've been waiting to see what you would do with a surgical tubing blaster, and it was worth the wait. This blaster is full of ideas that are begging to be incorporated into other builds. Great job.
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#5 Daniel Beaver

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Posted 01 July 2012 - 01:55 PM

Use a caster wheel for the pulley seems like a good idea - ought to impart much less friction upon the operation of the blaster (a big problem with pulley-based blasters).
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#6 Zorns Lemma

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Posted 01 July 2012 - 01:59 PM

How did you heat the PVC before bending it?
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#7 hamoidar

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Posted 01 July 2012 - 02:20 PM

How did you heat the PVC before bending it?

Seconded^

Also what is that tool that you used to bend the pvc for the SRSCB.
Anyways, great job! It's good to see people striving to make new things!
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#8 Carbon

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Posted 01 July 2012 - 03:02 PM

Also what is that tool that you used to bend the pvc for the SRSCB.

It's a tube bending spring. It just fits inside the tube to be bent, and keeps it from buckling/keeps it round during the bending process.

Edited by Carbon, 01 July 2012 - 03:03 PM.

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#9 cheyner

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Posted 01 July 2012 - 03:45 PM

Note that I am not sure if this will work with US materials like CPVC. My go to barrel material is the metric electrical PVC that they sell here which is much thinner, softer, and easier to bend than the CPVC that I have seen.


From this I think he is literally just bending it and it is holding its shape. Once bent and then mounted how he has, it has no room to return to its original shape. Us North Americans could probably do something like this using PEX, as it is the closest thing I can think of that is readily available, yet flexible. Stubs of PVC on the ends would give more reliable results then using just the PEX in couplers.

Edited by cheyner, 01 July 2012 - 03:50 PM.

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#10 KaneTheMediocre

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Posted 01 July 2012 - 04:27 PM

This looks really sweet. I'm especially interested in your hopper, as that's something I've wanted to try with PEX for ages, but never figured out a good jig to do it. The tube-bending spring is something I've never seen before, and I will definitely have to find one for my hoppers.

You're definitely right about the potential for air to slip around the dart in hoppers, and I bet they work in lower volume blasters than a wye hopper. I observed this with the CPVC hoppers I made here but these had a much sharper bend that I think was causing the darts to fly oddly.

If you wanted to reduce your deadspace, you could probably do so by using some combination of PVC and couplers to reduce the ID in the region where the surgical tubing remains at full contraction. This may or may not make any sense without a picture.
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#11 arfink

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Posted 01 July 2012 - 10:19 PM

Nice job.

Does the pully help decrease the draw strength needed otherwise by a 'straight pull?

What happens to the 'slack' of the cable when it's primed?


I think that the pulley's main advantage is that you actually get a reduction of friction from the rolling action of the pulley, as opposed to just dragging the cable over a bolt.
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#12 Crater

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Posted 02 July 2012 - 02:56 AM

It's not perfect, but it's pretty cool. Based on what I've seen of each, I think this is more deserving of the title "pinnacle of Nerf evolution" than the Hail-Fire. Every once in a while, something comes along that combines several innovations that would be cool on their own into one amazing package: DraganFlyer X6, PlayStation 3, MacBook Pro with Retina Display, and now The Dwarf. I wish everyone success in incorporating those innovations into their own (open source, in this case) products.
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#13 Parallax

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Posted 02 July 2012 - 11:04 AM

This is an awesome blaster boot, great job.

Lots of great innovation and explanation!
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#14 Meaker VI

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Posted 02 July 2012 - 01:24 PM

That hopper setup looks great. You can also bend pipe by packing it full of sand before heating and bending it. Might be more readily available than a specialty spring (which can also be fit outside the pipe IIRC). Note that I haven't tried that with PVC/CPVC, it's just common pipe-bending technique.

I like the use of a secondary pipe for the charging action.

I'm going to need to implement some of this into my next build, I was just thinking this weekend about how to shorten a 24" (body, not including barrel) newish-style pump snap and cables were the only way I could come up with without shortening the plunger (I've got a short-draw blaster design that I'm still fussing with to get working right) or using a reverse-plunger (which I don't like).

A totally separate side discussion - but how did you cut your slots? They look pretty good, and I'm currently looking for better/more efficient methods of doing them. The one I most recently learned (and used to great success) is using a piece of Manson's twine as a saw. Worked great, just really slow; apparently a wire saw would work better, but I didn't have a suitable wire when I tried it.

Edited by Meaker VI, 02 July 2012 - 01:25 PM.

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#15 cmeej

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Posted 02 July 2012 - 06:20 PM

Fugliest thing I've ever seen. Fantastic job. I think I want it's babies.
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#16 Boot

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Posted 02 July 2012 - 08:21 PM

Thanks for all the replies! It makes all the work worth it just to know that people will be incorporating ideas from this blaster into their builds in the future.

Does the pully help decrease the draw strength needed otherwise by a 'straight pull?
What happens to the 'slack' of the cable when it's primed?

This has more or less been answered by others, but I think I should clarify. The plunger cable is not wrapped around the pulley multiple times. The reduction in draw strength is comparative to simply dragging the cable around a bolt, and is a combination of using Carbons "revolutionary" (see what I did there ;) ) extension spring system.

I cannot believe I forgot to mention your second question in the original writeup... Thanks for bringing this up. The original plan was to use a small neodymium magnet to hold the primed cable in place, however one of the benefits of using a stiffer wire is that is roughly retains its shape when primed (since it would have to bend more to put out slack, and it naturally defaults to its more "comfortable" least bent position). This, combined with the end cap with small hole drilled in it means that any minor slack (of which there is never more than a few mm) is compensated for.

...I especially like your use of the surgical tubing...

Thanks (for both the compliments and the clarification on MiG's question)! I cannot stress enough the benefits of using surgical tubing. While some have said that it potentially dries out, I have never had this problem, and as long as the tubing is contained within the blaster it should be fine. Otherwise it effectively eliminated all plunger friction originating from places other than the plunger head itself, and provides a much smoother power to draw weight ratio.

Oh, I've been waiting to see what you would do with a surgical tubing blaster, and it was worth the wait. This blaster is full of ideas that are begging to be incorporated into other builds.

That is high praise, thank you very much. I can't wait to see what you will do with these ideas!

...

Thanks, I always thought that reversing plunger draw was the way to go for downsizing blasters, but it never seemed to be optimized enough.

How did you heat the PVC before bending it?
Seconded^

It's good to see people striving to make new things!

I didn't heat the PVC, at least for the materials I am using no heating is required. Also, I am a HUGE hypochondriac when it comes to plastic fumes, and so I would never feel safe heating PVC in my poorly ventilated workshop.

Finally, @ hamoidar: I will always be striving to make new things. Thats the part of this hobby that interests me most. Hopefully there will be much more coming throughout this summer.

From this I think he is literally just bending it and it is holding its shape...

You are absolutely correct. How it is mounted does not effect it however. You have simply to bend the PVC beyond it's elastic limit (in my case about 70 degrees) and the shape it back to your desired angle and it will hold more or less indefinitely. There could be no forward mount for the "SRSCB" and it would still hold its shape.

...hopper...bending spring...

...air slip...

...deadspace...

I'm glad to hear you'll be trying this out yourself, it was your wyeless hopper that actually got me thinking about looking at hoppers again. It is also good to hear that my understanding of the physics of a hopper is more or less correct, as I have only ever seen a hopper in action once...

Your solution to the dead-space issue did make sense without a picture, but I am not sure how big a difference it would make, since the narrower tube diameter would also limit the flow rate back into the SRSCB of the air that "overshoots" its target. The nice thing about this build is that if the ranges are unsatisfactory you have only to increase the draw length or tubing tension.

It's not perfect, but it's pretty cool. Based on what I've seen of each, I think this is more deserving of the title "pinnacle of Nerf evolution" than the Hail-Fire. ...DraganFlyer X6, PlayStation 3, MacBook Pro with Retina Display, and now The Dwarf. I wish everyone success in incorporating those innovations...

Wow, comparing my work to the DraganFlyer X6, MBP and PS3 is certainly very high praise, thanks a lot! You are right though, it is not perfect. If it were I would just share a picture and be done with it. The purpose of all explanation is, as you said, to let others incorporate those ideas into their own builds. Thanks and good luck!

...sand...

...charging action...

...implement into next build...

...slots...

I have heard of the sand packing technique, but I am not sure if it actually works when heat is not applied (although it may). If people are trying this with a thicker material like CPVC it may be and effective alternative technique.

I am very glad you like the priming action, and will be incorporating ideas from The Dwarf into your next build. The one thing I was a bit worried about was what people would think of the "bolt action" like priming system as opposed to the conventional pump action setup. It is also great to know that this writeup will actually be useful to other people, and will aid future projects!

Fugliest thing I've ever seen. Fantastic job. I think I want it's babies.

I was actually pretty proud of this builds looks, but then again my baseline for looks comes from my previous builds, which included this king of fugly. Thanks. (you may have to build yourself one to procreate with though, I don't think I would be comfortable providing you with this one).

I'm not going to respond directly to everyone because of the quote limit, but thank you all for your support and compliments!
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#17 lech

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Posted 10 August 2012 - 12:54 AM

Thanks Boot, your SRSCB is working beautifully for me, I'm using 16mm UPVC Conduit here in Australia, firing streamline length darts with heavy silicone tips. I had heaps of trouble firing these out of a hopper except with a BritHop, very loose barrel and using a high powered airgun(which yielded pretty poor results) but using your design they feed very well.

I first bought a pipe bending tool (pictured on the left) which kept crushing the pipe and generally being a pain in the ass(Not sure if I'm just terrible at using it or if its really not suitable for the task) but was enough to test the idea, and I bought a bending spring today which is much easier to work with. Using a Wye there is very little deadspace however I'm sure it works great with a 45+Tee as you have done. Please excuse the mess of electrical tape, its due to a discrepancy between conduit and PVC fittings.

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