Heckler & Koch HK417

The HK417 is a battle rifle designed and manufactured in Germany by Heckler & Koch. It is a gas-operated, selective fire rifle with a rotating bolt and is essentially an enlarged HK416. Chambered for the 7.62x51mm NATO round, it is intended for use as a designated marksman rifle, and in other roles where the greater penetrative power and range of the 7.62mm round are required. It has been adopted for service by a number of armed forces, special forces, and police organizations.

Design Details
The HK417 is similar in internal design to the HK416, but the receiver and working parts are enlarged to suit the larger 7.62mm round. The bolt is a 7-lug rotating type, which sits in a bolt carrier and operates in a forged alloy receiver resembling those of the Stoner-designed AR-10, AR-15 and M16 series weapons. 

Like the HK416, the HK417 is a gas-operated, with a short-stroke piston design similar to those found in the HK416, G36 and ArmaLite AR-18. The short-stroke piston is claimed to be more reliable than the original direct impingement operation of the AR-10 and AR-15 designs because, unlike these weapons, it does not vent propellant gases directly into the receiver, which deposits carbon fouling onto the bolt mechanism and is thought to induce malfunctions.

The early prototype HK417 used 20-round magazines from the H&K G3 rifle family, which did not feature a bolt hold-open device. Later prototypes, however, switched to a polymer magazine with bolt hold-open. The magazine resembles an enlarged version of the G36 series transparent magazine, except without the pins for holding more than one magazine together. In addition, a well proven 50-round, low profile drum magazine developed by HK for the HK21E machine gun can be fitted to the HK417 for use in support and sustained fire applications.