The SBR permit is required to alter any rifle so that has a barrel less than 16 inches long or an overall length of less than 26 inches. The suppressor permit -- often called a silencer permit -- is required to purchase or possess any device designed to muffle the sound of a firearm....
If a shotgun's shoulder stock is removed and replaced with a pistol grip resulting in an overall length less than 26" (even if the barrel is 18" or longer) the shotgun's legal status changes to Any Other Weapon (AOW). ... Under these regulationsthe Shockwave is legally considered a ...