The Ruger AC-556 was a variant of the popular Ruger "Mini-14" series of semi-automatic carbine rifles introduced in 1973. The Mini-14 itself held origins in the equally-popular M14 gas-operated Battle Rifle of the United States military, this development made possible through a post-World War 2 initiative that sought to replace the war-winning M1 Garand service rifle. As such, the M14 was based on the M1 Garand with the Mini-14, in turn, based on the M14. This then paved the way for the Ruger concern to finalize a militarized form of their civilian-minded Mini-14 offering, the design eventually emerging as the "AC-556". The AC-556 was primarily intended for sale to military and police markets and found a home in a few such groups of the world. Production of the rifle (since discontinued) was handled in-house by Sturm, Ruger & Company, Incorporated of Southport, Connecticut, United States.
The AC-556 retained its gas-operated, semi-automatic features and was chambered for the 5.56x45mm cartridge firing from a 20-round curved detachable box magazine. The system was given a receiver-mounted firing selector which allowed for a conventional single-shot, repeat-fire, semi-automatic fire mode as well as a three-round burst facility and full-automatic fire. Provision was also made for a bayonet mounting at the front of the weapon (near the forward sight) and the barrel was cleared to fire non-lethal rifle grenades for riot control. A flash suppressor was added to the business end of the barrel while a perforated glass-fiber handguard protected the operator's support hand from the generated heat of the barrel during firing. The design was strengthened where possible for the rigors of military service while retaining many of the proven quality features of the Mini-14 before it - including its notable compact profile.
Original AC-556 production versions were marketed with fixed solid wooden stocks. The later AC-556F and AC-556K models brought about use of a folding stock which collapsed under the weapon for a shorter overall length. These latter versions were, therefore, forced to install a full pistol grip under the aft-end of the receiver in the usual way. A thirty-round capacity magazine appeared for AC-556s for a short time.