Sorry, I guess the question was a little too broad. It was supposed to be generalized, but I have gotten curious about putting one on my shotgun. It's a Mossberg 500A.
Doesn't have any of the 'evil' features, so wouldn't it be legal?
Hell, even if it wasn't a folding stock, just a stock w/pistol grip would be fine.
good conversation going here, don't want it to go sour on my account