I have a Knoxx COPstock and it is quite amazing. No more gimmicky than the mercury thing really and you won't die if the mercury ruptures!! haha! Anyways, on the COPstock (folding stock), the whole reciever and barrel move back into the pistol grip. It aids cycling of the action actually. You seriously can shoot magnum slugs and buckshot ONE HANDED like a pistol with it. No kidding. The guy at Knoxx told me to make sure I did it, and I did, and it was cool. Kind of useless practically speaking, but shows how effective it is. I am not a fan of the recoil reducing full stock of theirs though, as it looks funny. Any of the tactical ones are good though, and don't change stock length. Cabelas is the cheapest place to order the COPstock, but maybe not for long, as they are selling for less than they are supposed to I guess.