LMT is the current supplier of the US Armed Forces for the M203 (under the AR15 or M16). These are available for civilian purchase, with NFA background check and of course the $200 federal tax
NFA is an abbreviation for National Firearms Act. Since 1934, the Federal Government has regulated the ownership of machine guns and several other types of weapons (including grenade launchers). The National Firearms Act of 1934 provides for the registration, and the taxing of the transfer, of a class of weapons described as NFA Title 2 weapons (sometimes referred to as "Class 3 weapons"). These include machine guns, short barreled rifles (i.e. rifles with barrels less than 16" or less than 26" in length overall), short barreled shotguns, silencers (also known as suppressors) and regulation of destructive devices such as grenade launchers.