The HK417's greater accuracy, effective range, and penetration offset its greater expense, its lower rate of fire, and its smaller ammunition capacity both in magazine and carriage. Purchasers of the HK417 have typically intended it to complement lighter assault rifles chambered for less powerful intermediate cartridges (often 5.56×45mm NATO), for the designated marksman role. The magazine resembles an enlarged version of the G36's transparent magazine, except without the pins for holding more than one magazine together. Later prototypes, however, switched to a polymer magazine with bolt hold-open. The early HK417 prototype used 20-round magazines from the Heckler & Koch G3 rifle family, which did not feature a bolt hold-open device. The short-stroke piston may be more reliable than the original direct impingement operation of the AR-15 design because, unlike these weapons, it does not vent propellant gases directly into the receiver, which deposits carbon fouling onto the bolt mechanism as well as heating it up.
Like the HK416, the HK417 is gas-operated with a short-stroke piston design similar to that of the Heckler & Koch G36.
The bolt is a seven-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 rifles. The HK417 is similar in internal design to the HK416, although the receiver and working parts are enlarged to suit the larger 7.62×51mm cartridge.
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