That's an ingenious design, and I hope you can make it work.
In my mind, if you want to use high pressure gas, you should move away from the paintball area, and look into industrial gases, which are often available locally even in small towns (although you'll likely have to order the stuff). The smallest cylinders run at around 2300-2500 psi, and could easily fit inside a backpack. The reason that I suggest this is because paintball regulators are specially designed for paintball guns, whereas industrial gas regulators usually go directly to NPT threads on the outlet.
Specifically, I would look into nitrogen, as it is easy to obtain, doesn't leak as readily as helium, doesn't randomly explode like hydrogen, and the regulators are relatively cheap (and commonly range up to 500 psi on the output side).
Failing that, I suppose HPA would be your best, nonflammable, bet. I personally would stay away from any liquified gas on a semi/full auto gun, due to cool down issues.
On the "convincing your parents" front, I don't think many people are too afraid of helium, as it sounds very friendly and balloonlike. Nitrogen sounds kind of bad because of the similarity to "hydrogen", which most people would identify as being bad (if you value you're life, don't use H2 as a pressure source). Mentioning the word propane should be avoided completely. Also, avoid mentioning the word "gas", as some people are scared of "gases" (in yet they continue to breathe
). The entire process is politics, made all the more difficult by talking to people who don't use reasoning, and don't change their minds, even in the face of overwhelming proof.
Good luck on the entire project, and remember, designing and building a full/ semi auto, blowback operated airsoft rifle with only rudimentary tools and supplies is nothing compared to convincing your parents to let you buy the parts.
