There's no max barrel length for a pistol, just don't attach any kind of buttstock or vertical front hand grip.
Every piston AR I've seen has the bolt carrier moving into a buffer tube so most piston kits wouldn't help with eliminating the buffer tube. Also it would have to be an extremely short piston system for a pistol. I've never heard of such a thing.
Last edited by robrob; 12-14-2010 at 08:51 PM.
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