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BullPAC


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#1 KaneTheMediocre

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Posted 10 June 2012 - 05:33 PM

edit: Later version of bullPAC looks like this, changes described in replies below.
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This is my bullpup, pump action crossbow.
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I built this because I wanted to make PACs more compact. It has 5" of draw, and can be made to shoot as hard as a standard PAC with about twice the bow tension of a standard PAC. So there's certainly room for improvement on the efficiency front, but it does work and is merely the size of a large shrubbery.

Using bow arms in the front pretty much requires a forward plunger rod motion, and putting that in the back means pulling, not pushing, the plunger head. So, I need to pull the rod through the front of the PT volume, which necessitates a slippery, airtight rod seal. My rod seal is neither of these things, (Hence some of the inefficiency) but it works:


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Yep, that's a blob of polyethylene with a hole in it. Plunger rod is 1/4" steel, polished with a dishwashing sponge. I accidentally used music wire for my plunger rod--Be more careful than I at the hardware store.

Plumbing out of the PT was done in the time honored tradition of loosely held shortened fittings crammed into each other in ways not advised by the manufacturer, and then covered in hot glue.


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The seal is a lathed blob of polyethylene with a skirt seal on it. Set screws hold it in place on the plunger rod. I was too lazy to do a check valve.


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The catch is a lever snapoid, inspired by my earlier adventures in reverse bullpups
http://www.nerfrevol....php?f=9&t=2779
and Atomatrons ERTL
http://www.nerfrevol...&p=47073#p47073

The handle was made out of 3 layers of 3/8" LDPE sheet. A few layers of paper between the layers of LDPE on the nonmoving parts of the handle ensure a loose, slippery fit for the trigger made of the same sheet. Handle is secured with countersunk screws through the top, and the snap-on clamp (which is screwed in place).

The catchspring is held from the inside with a curved nail through the trigger, and into a hole in the handle. The spring is held from the outside and secured to the handle by aforementioned hole, with hotglue at the back. Hotglue is backed with duct-tape and a paper clip.

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A priming grip guide has been added that's not in this photo.

The plunger rod affixed portion of the catch is another lathed blob of polyethylene. It does exactly the same thing as the 3/4" endcap in a SNAP. It's secured with 1/4" shaft collars, due to poor planning and laziness. The shaft collars work great, but they're ~5 bucks each.

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The priming grip has a cutout so that the string can be pushed past the front of the handle. The orientation of this cutout needs to be correct to prime the blaster, so I added a 3/4" PVC guide hot-glued to the priming grip so that it couldn't rotate.

Edited by KaneTheMediocre, 03 March 2013 - 09:14 PM.

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#2 HasreadCoC

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Posted 10 June 2012 - 07:04 PM

I really, really, like this bull-pup design. I'm typically more of a spring-instead-of-bow-arms kind of guy, but it's easy enough to see how to make this a springer, as ryan has done already. After going back and reading your previous bull-pup design with the string/pulley priming, I can say that the parts of this revision I like best are:

-No messing around with spring tension. Music wire might not be the best choice, but I'm sure we can use either 1/2" nylon and a rainbow catch, or whatever that metal rod ryan used was. In any case, a solid dependable plunger rod has me quite enthusiastic about this design.

-No retardedly long barrel coming out of the wye. This puts the whole wye part closer to the front, enough that it still has the shortness of a bull-pup, without requiring a barrel needlessly long and full of deadspace. I know it could probably have been shortened on the last design and just stop part way down the blaster, but it still irked me. This design fully fixes that.

-Way more streamlined and slender looking overall (minus the bow arms part lol).

Only thing I don't like as much about this particular blaster is the lack of a rainbow catch, but that can be done easily enough I suppose.

All-in-all, a fantastic piece of work.
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#3 Bchamp22795

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Posted 11 June 2012 - 10:30 AM

Where do you get your polyethylene blobs?
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#4 Guitarzan

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Posted 12 June 2012 - 01:21 AM

im new to the homemade scene and my opinion doesn't count for much but awesome job! I always enjoy reading stuff made by inovators rather than duplicators.

I'm disappointed to hear that such a well designed blaster is so inefficient though.
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#5 KaneTheMediocre

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Posted 02 August 2012 - 09:55 PM

edit: See also https://sites.google...jects/bullpac-3

 

 

2012-07-09_00-43-24_315.jpg

So, I used the parts of the rubber band thing to make this. Almost everything was recycled, including the slotted tube--Which was cut, and then extended on the other side with an internal coupler. The main change was getting rid of the 5x catch, and making a new plunger rod out of aluminum. Needs some ergonomic tweaking, but it works as efficiently as my other PAC.

edit: The hopper is of the homemade conduit bend variety. They work with rubber domes, but are terrible to make.

This is the rubber band thing disassembled:
2012-06-22_23-06-10_291.jpg

This is the rubber band thing assembled:
2012-06-22_22-56-09_699.jpg

The rubber band thing did shoot the 80-100' that I wanted without too much tension, but it was way too long in the stock and overall. The efficiency did not suffer much from friction, or poor seal, but the plunger mass was enormous. A good rubber band bullpup is certainly doable with some design changes, but at the moment I'm more interested in a better bullPAC so I scrapped it.

The catch:
2012-06-11_00-26-49_850.jpg

The plunger-rod-mounted part of the catch has 5 spots to rest, but that's not really as novel as the trigger / handle-mounted-catch-dongle

The not-quite-a-gun-yet
2012-06-11_00-40-18_572.jpg

This is the rubber band thing disassembled:
2012-06-22_23-06-10_291.jpg

This is the rubber band thing assembled:
2012-06-22_22-56-09_699.jpg

The rubber band thing did shoot the 80-100' that I wanted without too much tension, but it was way too long in the stock and overall. The efficiency did not suffer much from friction, or poor seal, but the plunger mass was enormous. A good rubber band bullpup is certainly doable with some design changes, but at the moment I'm more interested in a better bullPAC so I scrapped it.


Edited by KaneTheMediocre, 01 December 2015 - 09:24 PM.

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