What I was testing out in this gun was how to make a internally sprung plunger that used a firing pin. This is the plunger assembly:
The design came about after discovering that 3/4" endcaps fit just about perfectly in 1 1/4" PVC, with some minor edge sanding. The plunger is two endcaps (no O-rings) with a three or four inch length of tube. The dowel is attached to the bottom cap of the plunger with a cotter pin. I wans't able to find one long suitable spring, so I used two, separating them with a washer, plus washers at either end. Then, the spring assembly was capped with a 1 1/4" endcap. The hole for the dowel is very close to avoid rattles, so I drilled several vent holes in the endcap. The works is currently lubricated with a touch of vegetable oil.
I didn't want to even try a complex firing pin assembly, so I did something a lot simpler:
The firing pin is based off of something I saw on the Wikipedia page for rubber band guns, only upside down; the clothespin is glued down, and a hole drilled through it, into the plunger tube. I put in a nail, trimmed to the proper length (this took trial and error), and covered it with a dome of hot glue. The gun can then be fired by pressing the clothespin.
This is a closeup of the trigger assembly - once again, keeping it simple:
I drilled a hole in a piece of workable metal (this is a PCI slot cover), and slipped the firing pin through it. Pulling on the trigger lifts the pin just enough to release the nail.
I then cut a strip out of 3/4" PVC , and slipped that over the trigger assembly.
The barrel is 3/4" PVC with a nested Crayola barrel to keep darts out of the plunger. Fires stefans and micros (sonics work rather well). The barrel is fitted into a 1 1/4" --> 1/2" PVC bushing. The part is nice - the inside is shape into a half sphere, so it channels the air nicely. I'm using the hollow space in the handle to store the ramrod (1/2" dowel, stored in some 1/2" CPVC I have glued in there).
RANGE: The gun currently gets about 40-50 feet.
FUTURE PLANS: Well, get some O-rings on the plunger, for one. I've been having trouble finding them in the kinds of sizes or thicknesses I've been looking for. I'm also going to replace the dowel with an aluminum tube (the dowel tends to bind up a little). Cosmetically, I'll be painting it black, and probably adding a cosmetic clip/dart storage tube on the side to play up its similarities to a Sten. I'm having fun, I'll keep playing with it.
Edited by Carbon, 19 April 2015 - 04:16 PM.