Bullpump

Blaster Design and Instructions by KaneTheMediocre

Forum Threads:

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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/forums/index.php?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
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