I just got an Ender 3 Creality 3D printer and I've been thinking about fun things to build with it. Today I did some research about 3D printing a paintball gun and I have yet to come across even one complete model. Other than the components that must maintain high pressure, some mechanically necessary internals, and possibly (probably?) the barrel, it is beyond me why no one seems to have bothered. My first thought was designing one that would use multiple CO2 canisters for more time between replacing them, but since 3D printed parts supposedly cannot be airtight, that may not be possible. I question why, however. PETG handles for best paintball gun under 500, so pressure quite well as far as I know, and simply buying any small piece with a tight enough tolerance for the CO2 canister to screw into without leaking air and then using an airtight bonding agent that can bind this piece to a 3d printed chamber that had a hole about big enough for said piece seems like it would solve that problem.

Multiple canisters aside, (though possible using this method), the next issue would be having an airtight piece to release pressure from the chamber. Literally anything that could be mechanically released from the hole in the chamber which it was plugging by a trigger and imediately pushed back in, (by a strong spring, for example) would work. That's just one idea though. If that was the best idea I could have on the spot, surely someone with any experience in this could have a more nuanced one that would be more feasible in practice.

I would suspect the reason you don't see a complete model of an airsoft gun is multi-faceted.  Probably the biggest issue is most places that host 3d print models do not allow functional firearms of any kind for 3d printing, even airsoft guns.  The legality of 3d printing a functional gun varies from country to country because the parts can not be detected by metal detectors.  Also, the type of polymers involved in most 3d printed items may not hold up to the forces required under the pressures of a firearm in some cases, so liability also becomes an issue.  If you offer a complete 3d printable airsoft gun, in its entirety, then it malfunctions and injures someone, you could be held liable.  So I just think the legalities involved in those kinds of models might be why it's difficult to find in its entirety.