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1.
Made by Ruger and externally looks similar to a Mini-14, but is quite different internally. Came in two basic configurations: 13" folding stock model and the full stock with 18" barrel.

There is a switch on the rear of the receiver that permits the gun to be fired either: semi-auto; full-auto; or three round burst. AC stands for Automatic Carbine and "556" represents the caliber of the round it fires. 5.56mm. The 5.56 round is a military cartridge, loaded hotter than .223 and has slightly different dimensions than .223. You can fire either 5.56 or .223 out of the AC556, although it's designed to cycle with 5.56.

The US military has never adopted the AC556. Bermuda is rumored to be the only military that has purchased AC556's for military purposes. Police departments and prisons have made major purchases of the AC556 in the past, but have switched to the M16 as it has become more mainstream and reliable.

Depending on where they live, US civillians can purcase and own the AC556. It requires that the buyer make application on a BATFE Form 4, undergo an FBI background check and obtain a CLEO sign-off before the machinegun can be transfered to the buyer. A Class III firearms dealer must facilitate the transfer. A $200 transfer tax, must be paid and tax stamp issued to the buyer by the Treasury for the machinegun to be registered to a new owner. There is no "license" for owning a machinegun. Only someone who already has a Federal Firearms License (gun dealer) can apply for and receive a Class III license (let them deal in machineguns and other NFA items)
It took me 2 months to get my CLEO to sign off on my Form 4, and once he did that it took BATFE 3 months to approve my application for transfer. Then I went down to my Class III dealer and picked up the AC556 I paid for 6 months ago.
by Ricardovitz September 17, 2009
 
2.
The select fire version of the Ruger Mini-14 carbine. The 556 represents the caliber of the round it fires. 5.56mm also known as the .223. The AC in the name stands for automatic carbine,(AC556) The rifle has a selector switch on the receiver to select from fully automatic, three shot burst, or semi automatic. These rifles are sold strictly to Military and law enforcement, or an individual who can obtain a Class III firearms license.
The AC556 is a fine firearm.
by Freakshow 1975 October 28, 2007