The firing system uses a short-stroke piston driving an operating rod to force the bolt carrier to the rear. This design prevents combustion gases from entering the weapon’s interior, a shortcoming with direct impingement systems. The reduction in heat and fouling of the bolt carrier group increases the reliability of the weapon and extends the interval between stoppages; which is noted to usually be "100% better" than an M-4, or twice as much. It also reduces operator cleaning time and stress on critical components. The SA80 bull-pup assault rifle and the AR-18 used this system long before the HK G36 was developed. According to H&K "experience that Heckler & Koch gained during its highly successful 'midlife improvement programme' for the British Army SA80 assault rifle, have now borne fruit in the HK416."
It essentially is a gas-operated weapon that uses burnt powder gases from the barrel, bled through a vent near the muzzle which transmits the gas thrust to the bolt carrier, providing automation to the moving assembly; it fires from a closed bolt position. The weapon uses a self-regulating spring-buffered short-stroke gas piston system (the rifle has no gas valve). The rotary bolt features 7 radial locking lugs and its rotation is controlled by a cam pin guided inside a camming guide cut-out in the bolt carrier. The bolt also houses a spring-loaded casing extractor and an ejector.
The bolt is automatically locked to the rear when the last round is expended, but the bolt catch can be deactivated. The bolt catch button is located at the forward end of the trigger guard. The spring-loaded folding cocking handle extends forward in line with the barrel of the rifle (it is located on top of the receiver, under the carry handle). It can be swung to either side of the receiver, depending on whether the user is right or left-handed and is locked when pressed inward. When locked at a perpendicular angle to the receiver, the handle can be used as a forward assist to force the bolt into battery, or to extract a stuck cartridge casing (the cocking handle's design is protected in the US by patent number 5821445, by Manfred Guhring).
Spent cartridge casings are ejected through a port located on the right side of the receiver. A brass deflector keeps cases from striking left-handed operators in the face. There is no ejection port cover as the bolt closes the ejection port to particulates when it is forward. The weapon features a hammer-type striking mechanism.
The gas piston is of 3-piece design to facilitate disassembley, with a hollow forward section with 4 radial gas vent holes fitting around a stainless steel gas block projecting rearwards from the foresight housing. The gas is vented from the barrel and travels via a vent through the foresight housing into the hollow front section of the piston, which causes it to move rearwards a short distance. The rear end of the piston emerges through the barrel extension to contact the forward face of the bolt carrier, causing it in turn to move rearward. The bolt is moved into and out of the locked position via a cam pin that engages a helical slot in the bolt carrier, which rides on two metal guide rods (each with its own return spring) instead of contacting the receiver walls, providing additional clearance for foreign matter entering the receiver.
