1) useless and provides 0 benefit unless used with the folding stock. 2) folding stock makes it an NFA item requiring a $200 tax stamp 3) they tear the shit out of your slide, even shown if you look closely at the 2:56 - 3 minute mark. 4) expensive, buy a pistol caliber carbine for the same money with more usefulness 5) many versions from several makers are currently availible and none are popular for the reasons I just mentioned plus more