Carbine Length: Built for combat reliability in all conditions, the carbine length gas system is the military standard for the M4 Carbine. Most M4 Carbines utilize this gas system length. This gas system is the only option for barrel lengths ranging from 10.3” to 12.5”(5.56MM). It is also available in 14.5” (military standard) and 16”(5.56MM). It is the only option for a 16” 300 BLK barrel.
Mid Length: Mid Length gas systems are growing in popularity each year for softer felt recoil than Carbine (given congruent barrel length, caliber, and muzzle device). It can also hinder wear on the extractor. It is available in 14.5”-18” barrel lengths (5.56MM). Concerning 16” and 18” 6.8 SPCII barrels, mid length is the only option.