 These can either be simple shoulder stocks fitted to a pistol or full carbine conversion kits, which are at least long and replace the pistol's barrel with one at least long for compliance with the US law.In the US, fitting a shoulder stock to a handgun with a barrel less than 16" long legally turns it into a short-barreled rifle, which is in violation of the National Firearms Act.Under the National Firearms Act of 1934, firearms with shoulder stocks or originally manufactured as a rifle and barrels less than in length are classified as short-barreled rifles.