The You Tube clip to which I am referring is not of the gun firing. It shows the gun from all angles so should provide better detail for you. The AAC weapons were never adopted by the United States forces. Any "M" number would either be a manufacturer's designation (as in the "M" model aircraft cannon) and not reflective of U.S. military adoption. BTW, the M-2 submachine gun was adopted by the U.S. Army. It was superceded by the M-3, also a Hyde-Guide Lamp design. The M-2 was delayed due to production problems by Marlin Firearms, the prime contractor for the production of the submachine gun.