U.S. citizen ownership

Sales of select-fire or full automatic M4s by Colt are restricted to military and civilian law enforcement. Only under special circumstances can a private citizen own an M4 Carbine in a select-fire or fully automatic configuration. While many machine guns can be legally owned with a proper tax stamp from the Bureau of Alcohol, Tobacco, Firearms and Explosives, an amendment to the Firearm Owners Protection Act of 1986 barred the transfer to private citizens of machine guns made in the U.S. after May 19, 1986. The only exception was for Special Occupational Taxpayers (SOT): licensed machine gun dealers with demonstration letters, manufacturers, and those dealing in exports and imports. As such, only the earliest Colt M4 prototypes built prior to May 19, 1986 would be legal to own by civilians not in the categories mentioned. However, as US firearms law considers the lower receiver of a M16/M4 type rifle to be the "firearm" (the serial numbered and, in the case of machine guns, registered under federal law, part of the weapon), a registered Colt M16 (much more common than an actual M4) may be configured as an M4 by replacing the M16 upper receiver/barrel assembly with an M4 top half, and replacing the fixed rifle stock with a 4-position telescoping M4 stock.