The Colt Advanced Piston Carbine or Colt APC (internal product number P0923) is a lightweight modular 5.56mm caliber piston-operated, magazine fed carbine with a one-piece upper receiver which is capable of firing in automatic and semi-automatic modes.
The APC was adopted in limited quantities by several military and law enforcement agencies.
To improve reliability, Colt used an articulating link piston (ALP) which "reduces the inherent stress in the piston stroke by allowing for deflection and thermal expansion".
[1] In traditional gas piston operating systems, the force of the piston striking the bolt carrier can push the bolt carrier downwards and into the wall of the buffer tube, leading to accelerated wear and even chipped metal.
The ALP (articulating link piston) allows the operating rod to wiggle to correct for the downward pressure on the bolt and transfers the force straight backwards in line with the bore and buffer assembly, eliminating the carrier tilt.