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The Bullpump

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#1 KaneTheMediocre

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Posted 13 May 2011 - 08:03 PM

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So that's what the current version looks like. It's pump-action, with 5.5" of draw, and it shoots about slugs 100' flat out of a hopper just like every other homemade these days.

But this was not the first version. I learned a lot from the 3 prior versions, each of which was to some degree cannibalized for parts later. First, there was a simple, string-primed rodless springer:

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My original plan for this was to prime it by pushing the plunger back with a rod in the front, but I got less ambitious and just used a string instead. The main innovation with this blaster was the catch / plunger, which used a polycarb lever trigger that directly catches on the plunger head. The plunger head design has not significantly changed since this first iteration, and is conceptually very similar to the SNAP plunger. The main difference is that in order to use a skirt seal, I needed to have a 7/8" tube around which to put the skirt. The easiest way to get that was 3/4" CPVC, so I used that for the main body. The skirt is contained between an endcap and a cut piece of coupler. The catchface of the plunger is a 3/4" PVC endcap, which I had to bore out to fit over a 3/4" CPVC endcap so that it fit on the body of the plunger head. *********I'll add in a paint diagram later, because this is somewhat important.*********

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It's very important to note the shape and position of the trigger/catch and the plunger vs the trigger pivot location. I didn't realize this at the time, but the plunger exerts a torque on the trigger/catch in the wrong direction--that is, it's pushing the catch out of the way. This is why I needed such ridiculous rubber banding in order for the catch to hold. It doesn't help that there's no good place to put a compression spring. It's surprisingly not that bad of a trigger pull, although it's much worse than the average nerf gun. Also note that the front of the catch is gently sloped--The plunger itself doesn't have any real slope to it, so this is critical to allow the spring and the plunger to be pulled past the catch point.

Another curious bit about this blaster is the spring, which was taken from Ryan's box o' springs. At the time I thought it was a K14, and I ordered some for the next versions. It's not a K14, but I was lucky enough to guess the right spring on my second try. **********add a mcmaster number here***********. Since there's no plunger rod, the spring has to ride on the inside of the plunger tube.

After building v1, I realized that the string gave me all kinds of options for priming mechanisms. So, I decided to do a bullpup version, so that when the barrel was included, I could have something more compact than any other homemade pump action blaster. This is what happened:

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This is the smallest of the bullpups that I made--About the size of a barrel-less +bow. The plunger is facing backwards, but the string does a U turn around a U-bolt at the front of the blaster. The reason for this is ergonomic--Ryan made a bullpup

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blaster that did not feature this direction reversal, and although I loved the bullpup structure, the "push-action" priming motion was very uncomfortable. However, I did a piss-poor job of it. Although I initially blamed the absence of a pulley for the ruinously difficult priming of this blaster, the actual problem is that the priming handle isn't in-line with string that it's pulling on. So, when you prime the blaster, you exert a net torque on the handle, which causes the handle to rub extra hard against the plunger tube. Also, I just BARELY made it long enough to prime, so unless you held the priming handle very oddly, you would ram your hand against the front of the trigger handle. Neither of these first two blasters had remotely smoothed edges on the handle, so priming and firing this basically required stabbing both of your hands with polycarb corners.

The U-bolt that reverses the direction:
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The new trigger assembly (pictured below) is worth a closer look: The most obvious change is going from .420" polycarb to 1" wood, which was done for simplicity and ease of machining. However, the more interesting change is the direction of the slope and catch on the trigger. In the bullpup configuration, the torque on the trigger has reversed, and so the catch is now self-restoring, and holds without any catch-spring or rubber bands (The rubber bands are still used to make the trigger move into position automatically).

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My third attempt was focused on two main problems with version 2: The blaster wasn't quite long enough for the priming handle to be comfortably used, and the priming action was very difficult owing to the aforementioned torque problem. Also, at the time I thought that the U-bolt friction was contributing to the difficult prime as well, so I used pulleys to solve both problems:

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The result of these changes was the first war-worthy blaster in the series, although it was never really finished. The priming action was nice and smooth now that you pulled on a string that was in-line with the handle, and the blaster was still very short by pump action standards if you include the barrel in the measurements. Other incidental changes included switching the handle and trigger to 3/4" wood, and making a massively robust snap-handle clampy pipe thing. This unfortunately required dremelling a massive hole for the trigger, but it was much more stable than the two mini-clamps used earlier, and allowed me to easily screw the handle into the plunger tube without putting holes in the pressure-holding regions of the blaster. Aside from those changes, this handle is really not very well made, it's got way too much slope for the trigger. It's not horribly uncomfortable, but it's not terribly nice either. Although this isn't the final version, I think it's worth further development as a variant of the bullpump design.

Another change made at this phase was ditching the rubber bands for a tiny, very weak, catch spring. Now that the trigger has properly restoring torque, that's all that I needed. A nicer mounting system for it is needed, but I'm OK with duct tape for now:

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The next change I made was largely inspired by Beaver's new pump action snap

http://nerfhaven.com...showtopic=21024
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The pulley system from before was replaced by a much simpler bolt-with-nylon-bushings-on-it, and instead of a vertical foregrip, a traditional horizontal foregrip was used. This worked great, but getting the knot in the right place required a lot of trial and error, and each time I moved the knot I had to spend 1/2 hour putting things back together. To make matters worse, the loop-knot required for this design kept slipping! I tried a variety of means to keep it in place, including (but not limited to) Tacky-glue, hot-glue, flame, prayer, pulling it SUPER tight, and intense man-on-string negotiations.

The solution to these problems was to add a 3/4" endcap. The endcap had holes through the side for the priming handle bolt to pass through, and two holes on the end. One larger hole was used to allow the string to pass and freely move through the endcap on it's way to the turn-around bolt, and one smaller hole was for the end of the string, which was knotted in the usual single-hitch manner of string-stops and bowstrings. In the process, I lost some of the nylon bushings, and so I just went on without them. I found the blaster was no harder to prime than before, so I won't bother with the nylon bits in the future.

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One semi-critical tweak that I discovered in the final version was the spring orientation. The spring used did not have closed ends, so precautions need to be made to ensure that the loose end of the spring doesn't get caught in the catch hole as the blaster is primed. This is fairly rare, but it can occur if the spring is rotated such that the loose end is at the bottom. So, I taped the spring to a notch on the spring rest, oriented the spring in the opposite configuration, and put in a couple set screws to further contain the spring once the tape fails.

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That's all for now. There's more info that I'd like to add, and I do intend to do a normal writeup once the bullpump design has been war-tested and tweaked. Hopefully this non-writeup has been useful and/or educational to a few nerfers.

Current TODO list 5/16/11

1. Put cover over front to soften it and prevent the string from sticking out.
2. Add flap valve to the plunger head.
3. Make a comfy handle.
4. Add material to shape the foregrip
5. Make catch spring not use tape

Edited by KaneTheMediocre, 16 May 2011 - 09:18 PM.

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#2 Kid Flash

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Posted 13 May 2011 - 08:37 PM

Looks pretty neat. Almost reminds me of the larami ball blaster.
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#3 shardbearer

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Posted 13 May 2011 - 09:39 PM

Been waiting for a better version for a while now. This looks like it needs some slimming down and optimization, particularly around the handle and whatever is right under the wye, but it all looks really nice. I understand how the 3/4 endcap attached to the handle works, but not how it is any better than simply tying it to the bolt. If I were to do this I would put a bunch of loops in the string, maybe 1/4" apart, and just switch which one the bolt was through to adjust it. You claim this blaster have the same range as every other homemade [k26] springer with a hopper, 100', but I thought they normally went 120'. Maybe thats without a hopper or with slingshots. Which spring did it end up being? Stronger or weaker than a [k26]?

EDIT: Sorry, but this is not actually a bullpup.

Bullpups are firearm configurations in which the action is located behind the trigger group and alongside the shooter's face.


Edited by shardbearer, 14 May 2011 - 11:41 AM.

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#4 Y-Brik

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Posted 13 May 2011 - 11:00 PM

Very nice. After messing around with some pump action designs, I think the vertical foregrip is something worth keeping. Have you considered a mashup of the 2 styles? A vertical foregrip with a bolt that connects to the 3/4" endcap?
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As I said I have not not alot of testes yet but I will be once I finish the mod.

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#5 knexpert66

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Posted 14 May 2011 - 06:57 AM

This is really cool! Hope I can see it in person at Cataclysm 2.
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#6 blitz

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Posted 14 May 2011 - 07:21 AM

This is pretty cool. I like how it is pretty small, you don't have to worry about the hopper falling off and the reverse-pump-action.

It looks (deceptively?) easy to make. Might have to try my hands at this.
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#7 KaneTheMediocre

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Posted 15 May 2011 - 01:51 AM

Been waiting for a better version for a while now. This looks like it needs some slimming down and optimization, particularly around the handle and whatever is right under the wye, but it all looks really nice. I understand how the 3/4 endcap attached to the handle works, but not how it is any better than simply tying it to the bolt. If I were to do this I would put a bunch of loops in the string, maybe 1/4" apart, and just switch which one the bolt was through to adjust it. You claim this blaster have the same range as every other homemade [k26] springer with a hopper, 100', but I thought they normally went 120'. Maybe thats without a hopper or with slingshots. Which spring did it end up being? Stronger or weaker than a [k26]?

EDIT: Sorry, but this is not actually a bullpup.


1. The knot doesn't actually work, because it kept slipping. See original post.

2. It shoots hard enough. No one does accurate range measurements, and it's usually exaggerated. In any case, I'd actually be happier with this blaster if it shot significantly LESS hard than a +bow. The spring feels like it provides about the same amount of tension.

3. My bullpup may actually qualify by your definition, if the "action" is the feed mechanism. The analogy of firearm parts vs nerfoid gun parts really breaks down because they really have very few components in common. What I mean by it is that the bullet is fed from the rear of the gun (and as you said, behind the trigger and where you put your face if you want to really aim it rather than just point it), which maximizes the barrel length per size of the gun. This is, as far as I can tell, the functional consequence of the bullpup configuration in both contexts, but I'm not particularly knowledgeable about firearms. I also wouldn't mind re-naming it anyways.

Very nice. After messing around with some pump action designs, I think the vertical foregrip is something worth keeping. Have you considered a mashup of the 2 styles? A vertical foregrip with a bolt that connects to the 3/4" endcap?


To me, the main advantage of the vertical foregrip is that you can made it shorter than a horizontal grip. However, the force of your hand pulling on it isn't aligned with the force of you pulling on the string in this case either, so the handle gets torqued and the same friction problem comes up. You can compensate for this by making the top tube part of the handle longer, which results in a mechanical disadvantage on the force between the plunger tube and the priming handle, which in turn reduces the resulting friction. However, this defeats the point of requiring less length, which was the reason I had used the vertical grip in the first place.

This is pretty cool. I like how it is pretty small, you don't have to worry about the hopper falling off and the reverse-pump-action.

It looks (deceptively?) easy to make. Might have to try my hands at this.


Although the hopper itself will never fall off, clips (especially ridiculously large ones that I like to use) are probably about equally prone to falling off. So it's not that big of an improvement.

It's not that hard to build. There are a few cases where I used exotic tools, but in each of them another method or slight design deviation could be used to work around it. The rectangular hole for the trigger was cut with a dremel for the first two versions, and then a mill for the second two versions. There's no performance difference between the two, although using the mill was easier, cleaner, and made it look nicer. That hole doesn't even need to be rectangular either, an oversize circular hole could be drilled instead. I was just trying to minimize the amount that I cut out of the pipe.

The plunger head actually required a lathe to make the 3/4" PVC endcap fit over the 3/4" CPVC endcap. However, a conventional SNAP head should work fine with this catch. If you MUST use a skirt seal like I did, then you could simply use a bushing from 3/4" to 1/2", and put the skirt around the 1/2" PVC. It won't fit tightly, so you'd have to tape out the gap with e-tape (From .840" to .875"), but if it's smashed between sawed off couplers it should still get perfect seal and center itself nicely.

Edited by KaneTheMediocre, 15 May 2011 - 02:22 AM.

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#8 shardbearer

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Posted 15 May 2011 - 02:37 AM

2. It shoots hard enough. No one does accurate range measurements, and it's usually exaggerated. In any case, I'd actually be happier with this blaster if it shot significantly LESS hard than a +bow. The spring feels like it provides about the same amount of tension.

3. My bullpup may actually qualify by your definition, if the "action" is the feed mechanism. The analogy of firearm parts vs nerfoid gun parts really breaks down because they really have very few components in common. What I mean by it is that the bullet is fed from the rear of the gun (and as you said, behind the trigger and where you put your face if you want to really aim it rather than just point it), which maximizes the barrel length per size of the gun. This is, as far as I can tell, the functional consequence of the bullpup configuration in both contexts, but I'm not particularly knowledgeable about firearms. I also wouldn't mind re-naming it anyways.

2. I think we often forget this, and get obsessed with ranges. Thanks for reminding us all.

3. I think of the definition of bullpump as where the handle is in front of a significant amount of the action, reducing space, but you are right that in nerf bullpup has come to mean something exactly like this blaster, where the propulsion is below the barrel.

To me, the main advantage of the vertical foregrip is that you can made it shorter than a horizontal grip. However, the force of your hand pulling on it isn't aligned with the force of you pulling on the string in this case either, so the handle gets torqued and the same friction problem comes up. You can compensate for this by making the top tube part of the handle longer, which results in a mechanical disadvantage on the force between the plunger tube and the priming handle, which in turn reduces the resulting friction. However, this defeats the point of requiring less length, which was the reason I had used the vertical grip in the first place.


I think he meant that it would be better ergonomically, and it would be totally possible to put a tee over the current handle. Do you think this would be better?

Lastly, how polished would you say this design is? Would you sell these through MHA?

Edited by shardbearer, 15 May 2011 - 02:40 AM.

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#9 KaneTheMediocre

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Posted 15 May 2011 - 03:15 AM

I think he meant that it would be better ergonomically, and it would be totally possible to put a tee over the current handle. Do you think this would be better?

Lastly, how polished would you say this design is? Would you sell these through MHA?

It's certainly possible to make a T-handle that's the same length as the pump-grip that goes there now, and the friction would be manageable. Or you could screw on a handle to the pump grip.

I wouldn't sell a blaster exactly like this one, but I would sell something very similar through MHA. Mostly, it needs a nicer handle, and not to use duct tape to secure the catch spring. Depending on demand, I might not make them clear as I don't think that the clear looks particularly nice. The next prototype will be sales-worthy.

Edited by KaneTheMediocre, 15 May 2011 - 03:19 AM.

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#10 Noodleownz

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Posted 15 May 2011 - 09:57 AM

Kane, this is really cool. Are you thinking about adding a stock, if so, how are you going to attach it?
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#11 KaneTheMediocre

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Posted 16 May 2011 - 07:22 PM

Added a ball-nose plunger (3408A107) so that the priming grip stays forward while primed. It's not as clean as I'd like--There's a superglue mess because I tried gluing on segments of polyester tube to prevent it from oddly sticking out. Had I the foresight to do this prior to assembling the blaster, I think I would have been able to do this successfully, and tap the hole rather than forcing the thing to self-tap. I also might have been able to make the dimple the proper size, which would eliminate the 1/32" of play within the forward position. In any case, it's done, and it works fine.

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#12 lordnikon60

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Posted 18 May 2011 - 01:08 PM

I think that this blaster is grate. It looks easy to recreate but looks like it could be really powerful. I love it.
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