Adcor A-556

Lore & Background

Adcor Defense began producing upper receivers for the M4 Carbine under contract from Colt in 2005, manufacturing 1,000 uppers per day compared to Colt's 450. After years of production, Adcor decided to improve upon the weapon, claiming it was not a 'new and improved' AR but a patented revolutionary gas piston system with other key features. In early 2010, the A-556 was sent to H.P. White Laboratory for reliability testing; 6,000 rounds were fired in 120-round cycles with lubrication and cooling after 600-round intervals, and no parts were replaced nor any malfunctions occurred. The rifle then shot a .88 MOA group at 100 yards with 69-grain match ammunition.

The cornerstone of the A-556 design is its piston operating system, where a gas-driven piston is incorporated into the upper half of the forward rail system and never touches the barrel, making the barrel free-floating. The operating rod connects to the bolt carrier group and rides through a machined boss to eliminate carrier tilt. A spring-loaded dust shield on the bolt carrier and a port dust wiper are included. The forward-placed, foldable, non-reciprocating charging handle is mounted on the handguard and is ambidextrous, removable without tools to either side. The handguard has an M4 profile, so the charging handle sits directly above the rail, requiring unobstructed space on that side.

Production for the civilian market was announced on December 8, 2010, and the improved A-556 Elite variant was announced for public sale on June 27, 2011. In June 2012, a soldier was seen in Afghanistan with an A-556 upper receiver with a 10.5-inch barrel on an M4 lower receiver. The A-556 was entered into the U.S. Army's Individual Carbine competition on May 12, 2011, and the Elite version advanced to Phase II on May 4, 2012. The competition was cancelled before a winner was chosen; on July 17, 2013, Adcor announced it would not protest the termination and would focus on commercial customers.

Reader's Guide

The ADCOR A-556 represents a significant attempt by a former M4 component manufacturer to improve upon the M16/M4 platform through a proprietary gas piston system. Its entry into the U.S. Army's Individual Carbine Competition placed it among contenders vying to replace the M4 Carbine, though the program was cancelled before a selection was made. The rifle's design emphasizes reliability and accuracy, as demonstrated by its clean 6,000-round test at H.P. White Laboratory and its sub-MOA accuracy with match ammunition. The piston system, free-floating barrel, and anti-tilt bolt carrier were key features intended to address common AR-15 platform issues. The A-556 also introduced an ambidextrous forward charging handle mounted on the handguard, though this placement required unobstructed rail space on the side it occupied, a noted difference from designs like the Adaptive Combat Rifle and FN SCAR. Despite the cancellation of the military competition, Adcor continued production for the civilian market and released an Elite variant. The rifle's appearance in Afghanistan on a soldier's M4 lower receiver in 2012 indicates some field use, but its legacy is primarily as a commercial piston-driven AR-15 variant that competed in a major military procurement program that ultimately did not conclude.

Did You Know?

• Adcor Defense produced 1,000 M4 upper receivers per day for Colt, compared to Colt's 450 per day.
• The A-556 fired 6,000 rounds in testing with no jams, stoppages, or malfunctions.
• The A-556 Elite variant was selected into Phase II of the Individual Carbine Competition on May 4, 2012.