Gotya! To get 2-3 shots out of one 12 gram, you have to have a valve that opens and closes QUICKLY. Then an air chamber, then paintball regulator to control the pressure and you are over budget. You are probably stuck with a single shot.
This is Fastlearners "Pocket Rockets". Carries several clipped on his belt. He is VERY effective at eliminating my walking tank. Main Factor is I and my infantry cannot SEE the itty bitty popgun until AFTER he fires the first one. Well that is all to often, too late...
It runs off a 12 gram adapter but it is a singe shot. On 12 gram per shot.
It takes a while to reload so he just carries a couple. Hands one off to another player occasionally for double trouble for me.
See those pipe unions between the 12 gram holder and the barrel? That is the main valve. He sandwichs household aluminum foil layers in between the two halves of that pipe union as a BURST DISK. Controls the velocity by the number of layers of foil. By the way that little beast will shoot a 230 fps Nerf out of that VERY SHORT barrel.When it goes off, it sounds like a shotgun!
And that is the downside from my point of view. To get that kind of velocity out of a barrel that is that short, you have to have a very high operating pressure. A 12 gram at room temperature is about 1000 psi so GUESSING, it is expanding liquid CO2 into a very small volume will come close to matching that 1000 psi in that pipe. Since we are looking at PVC there, it makes me nervous. Basically, it builds pressure until something GIVES way and so far that has been the aluminum foil.
Many field require the guages, pressure relief valves, limit operating pressures to 150 etc. So make sure you will be allowed to use it.
Now up at Sherwood in LaPorte In, I ran into a BUNCH of pocket rockets of a different design. 12" Barrel, 1/4" Ball Valve, 1" Galvanized pipe air chamber. of various lengths up to a foot. Some were 12 gramers, other just hooked straight into a regulated air supply via remote hose. Range was limited to 20-30 feet. VERY Short Range but like I said, they had a BUNCH of them. I could not get anywhere NEAR their HQ or penetrate any mission into the area they were defending. They protected the perimeter around their HQ quite effectively. Even with infantry going with me, we would swat on or two, then another but just like mosquitos, there was ALWAYS another one waiting to bite. Again, downside. Galvanized pipes are rated 300 psi along with the METAL ball valves. The operating pressure would depend on the volume of the pipe but I'd wager they were past 300. But maybe not since their range was so low.
I asked Fastlearner what his opeating pressure was. He never put a guage on it. Ditto the guys I asked up at Sherwood.
In your R&D prototype for these toys, put a guage on it so YOU know what pressure you are using. Start LOW and work up to whatever your hardware is rated for. Once set, and if the field allows it, you can drop the guage in your field version.
Year ago we had a sort of contest for building the CHEAPEST anti tank weapon. A $10 Nerf gun. Sling shot. Crossbow. Fun thread. One you might be interested in, Budget WISE was 2" THIN Wall, schedule 20 PVC about 4 foot long. Had a screw across the pipe to stop the nerf from falling all the way through. The screw was about halfway down the pipe. Fins were trimmed so it dropped in like a mortar round. To fire it, you just swung it like a baseball bat. That Nerf would come flying out. Not much range but again, how many of them can you build for $100? Equip 30 guys maybe.
Chaw on them there ideas a bit and then swallow or spit.
You could replicate the Shorty for $150 and less if you can scrounge a used reg.
I spent to time pondering using the burst disk idea for an autoloading cannon. Each cartridge would be a PVC shell, burst disk with a Nerf inside. Just a gravity fed magazine. It didn't make it to the drawing board. Went another direction but let me know if you want to move forward and I share my ponderings. It has some potential for a pocket rocket. All you need is a holder for the cartridge that will seal good enough to build pressure behind it. Remote hose with ball valve for a "trigger" to presurize it.