self rifling pneumatic?

[psycix]

A sharp projectile could carve in a groove.
Now if that sharp tip is on an angle, the projectile will automatically rotate.

Its not a bad idea!
Maybe someone could create a piston with 5 (or more or less) slightly angled, sharp and sturdy fins on it.
The piston is airtight (o-rings) and the fins are actually a little bit larger in diameter then the inside of the barrel is.
Using pressure to push the piston through, the piston will rifle your barrel.
A new type of rifling machines!