For the U.S. special operations troops, Heckler & Koch created the HK416 as a significant upgrade over M4/M16-style rifles and carbines. The Heckler & Koch HK416 is a gas-operated assault rifle chambered for the 5.56×45mm NATO cartridge. The HK416 is the most dependable M4/M16 type weapon because it uses the HK-exclusive gas piston mechanism seen on the G36, which prevents propellant gases and carbon fouling from penetrating the weapon's interior. Models D10RS, D14.5RS, D16.5RS, and D20RS, among others, are available in a variety of barrel lengths (barrels from 10.4 to 20 inches) for the military and law enforcement market exclusively. In 2013, Heckler & Koch has used the A5 variation in place of the original HK416. This variety is improved, with a toolless adjustable gas regulator for suppressor usage and a remodeled lower receiver with ambidextrous firing controls. The HK416 A7, the next variation, has a KeyMod Rail System, which is lighter than conventional Picatinny Rail Systems. The HK416 A8 is almost the same as the HK416 A7, except it has a shorter hand guard, a steeper grip angle, and a height-adjustable shoulder support on the stock. The HK416 22LR, the final member of the HK160 family, is a semi-automatic rimfire variant of the HK416.