Another pump-action crossbow. It is superficially similar to my old crossbow
, but with a number of improvements.
Internals below (click the picture for a closer view). This is not a SNAP; it utilizes a ring-type catch, built from PVC endcaps, and is substantially similar to several other designs that have been posted. The meat of the plunger rod is a 1/2" wooden dowel, with the rear section of CPVC acting as a stabilizer and catch face. A little screw sticks out the bottom of the catch, which the trigger pushes against. With the exception of the handle, everything is easily acquired at your local hardware store. Still needs a stock.
I could not find workable catch spring for the life of me, so I jimmied up a compromise solution. It works very well, but looks funny.
The bow arms are offset by 45 degrees, to keep them out of my way. This setup offers a clear view down the length of the barrel.
I can't find any decent lubricants down here, so the blaster's performance is seriously hampered by friction. The catch performance seems quite good – I have had no misfires while playing around with it.
Found some lube, yay!
I experimented a bit with bow arm lengths, and 20" arms seems to be a sweet spot in performance, giving me ranges between 90-120ft (my darts are a mish-mash, hence the inconsistency). Much over 24", and performance drops way off. Under 18", draw resistance increases significantly, to the point where the catch spontaneously fails. These are 1/2" PVC arms, FYI, and pre-tension is kept to a minimum.
A note on bows:
Smaller arms will exert much more force due to a couple of factors. First, the deformation of the arms is greatly increased. Second, the force vectors are more parallel with the plunger rod.
Edited by Daniel Beaver, 22 February 2011 - 06:13 PM.