This product converts a pistol into a short barreled rifle (SBR) and is subject to NFA rules.
http://www.24hourtactical.com/Command-Arms-RONI-G226-For-Glock-Models-26-27-28-33-Black...

If you want a glack magazine compatible carbine Kel-tec's excellent little SUB 2000 works great.

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Of the Troops & For the Troops

I handled them at the NRA show. They are surprisingly sturdy. As Rob says, they convert a pistol into an SBRand must be Form 1'ed to be legal. Saying that, I'm pretty sure I've seen ads with them having 'pistol' buffer tubes vs the standard collapsible style. A guy could stick one of those SIG arm braces on there and be legal and have the same basic thing minus the collapsible part (the stock folds to the side)