pneumatic potato gun i made

[DonRotolo]

Exactly: A tight seal is a requirement.

Also, how much air are you pushing? It needs to be about 4 times the volume (at least) of the PVC tube.

Lastly, with 90 PSI, consider that the shock loads may cause the pipe to shatter. Two ways to make it safe, first encase the bottom 2/3 of the pipe in a larger pipe (no matter the size as long as it fits over the inner pipe), so the larger pipe will catch any shrapnel, OR second wrap pliable tape (like electrical, duct, or fiberglass packing tape) in a 1/4" to 3/8" layer around the lower 1/2 of the pipe, that'll slow down the shrapnel to nonlethal speeds.