This is my bullpup, pump action crossbow.
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:
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.
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.
The catch is a lever snapoid, inspired by my earlier adventures in reverse bullpups
and Atomatrons ERTL
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.
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.
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.