Cobras are awesome for a number of reasons. You can single them and have fun shooting things hundreds of feet away (NOT AT WARS, THOUGH), you can easily shotgun them, and you can fire large ammo with ease. While not an every-round blaster, they are especially perfect for Defend the Core. These qualities I wished to maintain.
For reference, see THIS LINK for internals. As you can see, it's a backpressure system, and a highly efficient one at that. The trigger pull lets out a tiny bit of air from behind the piston, and then 750 mL of pressurized air shoots out the front. That ratio of pilot air to actual air coming out the barrel is astounding.
As it turns out, this system is easy enough to replicate!
Materials (looking for cheapest sources, naturally):
(9") 3" Sch. 40 PVC (~$.30) (Obviously, initial cost of 3" pipe is greater than this, since you get it in 5 or 10 foot lengths)
(2) 3" to 2" reducing PVC coupling ($6.24)
(2) 2" to 1/2" PVC bushing ($2.62)
1/2" Sch. 40 PVC
(1) 3/4" PVC Elbow ($.34)
(1) 3/4" to 1/2" PVC reducing bushing ($.44)
(1) 1/2" to 3/8" Copper reducing coupling ($1.03)
1/4" NPT blowgun ($2-$6)
Bike Pump ($10 in my case)
Things you should already have, and last long enough to justify not including in the cost/incredibly cheap for the amounts used:
Let's start with the barrel. Simply hammer some 1/2" CPVC into the end of 1/2" PVC (roughly 11 inches of it), and stick a reducing copper coupler over it (1/2" to 3/8"). Easy enough. This helps keep everything straight, and gives a flat surface for the piston to seal against (alternatively, you could flatten the end of the plastic piping; my attempts at that failed, so I'v moved to manufactured fittings). Use epoxy to attach everything together.
Here's what we use to get all that volume. 1/2" to 2" PVC bushings, and 2" to 3" couplings. We'll use a short amount of 3" PVC to link both sides (about 3" worth). Just a warning; once you reach 3" PVC, more often than not, you will find foam core DWV piping that is not pressure rated. It might hold the low pressures we deal with, it might not. But be safe and find the actual pressure-rated stuff. So keep that in mind. Also, ignore the really short barrel. It's an old pic. Sorry.
Assemble all that together with PVC cement, EXCEPT for the back of the tank. Now we get to the part that requires you to do some judging.
Now for the piston and that bushing you have yet to attach: the small ridge of plastic on the inside of the bushing need sanded down, so that you can slide 1/2" PVC (with 1/2" CPVC hammered inside) all the way through. Put the cap on the end of the pipe, assemble without adhesive, and adjust until the piston only moves 1/8" to 1/4" (you can shove a pencil through the barrel, and measure its movement). 1/8" seems to achieve best results. If sealing is still a problem, then your fit is too loose. Wrap some e-tape around the PVC and lube it up.
Edited by Buffdaddy, 05 July 2011 - 12:12 AM.