[DonRotolo]

OK. Part of the problem is I can't see the sketch too well.

The air that enters the barrel is pressurized. That is the pressure I was writing about with safety and shock loads. As you continue, please consider what I wrote. PVC pipe might be rated for 150 PSI, but that's not a shock load - it might only withstand 1/2 that.....

To launch a projectile, you need to get a lot of pressurized air behind it. If the barrel is 80 cubic inches, and you have 8 cubic inches of air (about what a clippard tank holds) at 90 PSI, releasing it into the barrel changes the pressure to 9 PSI. Do you see how that works? (The calculations are actually more complex, really resulting in less PSI, but the principle is the same).

Now, if you release 80 cubic inches of 90 PSI air into the barrel, you end up with a total of 160 cubic inches of 45 PSI air. Start with 800 cubic inches of 90 PSI air and you get 880 cubic inches of about 80 PSI air... See the way this is going?

OK, now the other half of this is getting the air into the barrrel quickly. If you use a 1/8" pipe to move the air into the barrel, it'll take 3 or 4 seconds to let the air in, and that's much too slow. Use 3/4" copper plumbing pipe, and the difference is dramatic.

OK, that's enough for you to go on, let us know how far your half of a potato goes...

Don