The MG 3 is an automatic, air-cooled, belt-fed short recoil-operated firearm. It features a roller locked bolt mechanism that consists of the bolt head, a pair of rollers, the striker sleeve, bolt body and return spring. The bolt is locked securely by a wedge-like striker sleeve, which forces two cylindrical rollers contained in the bolt head outward, and into corresponding recesses in the extension of the breech of the barrel. On firing, both the barrel and barrel extension recoil to the rear compressing both rollers as they are cammed inward and out of their sockets by fixed cams, unlocking the bolt head. The bolt carrier and bolt then continue to the rear together guided by fixed guides while the barrel and barrel extension return to battery. Upon return of the bolt forward, the impact of the rollers against the camming surfaces on the breech carry the rollers from their seats, and, together with the surfaces on the striker sleeve, force the rollers outward, locking the bolt head into the barrel extension and ensuring a complete lock. The bolt also houses a spring-loaded casing extractor and ejector. Ejection is carried out when the ejector strikes the buffer head, sending a push forward through the ejector bar, which hits the ejector pin. This pin pushes the top of the base of the cartridge, which is still held by the extractor at the base, causing the empty casing to rotate and eject downward through the ejection chute.
The machine gun has an automatic-only trigger mechanism and a cross-bolt safety in the form of a button that is operated by the shooting hand (in its "safe" position the bolt release is disabled). The weapon fires from an open bolt.
The MG 3 feeds from the left side through a feed block using metal, 50-round continuous-link ammunition belts (which can be combined by cartridge) or disintegrating-link M13 or DM6 belts. In the light machine gun role, the DM1 MG 3 is deployed with a 100-round belt fitted inside a synthetic ammunition drum developed by Heckler & Koch that is latched on to the left side of the receiver. The rear wall of the drum is transparent and serves as a visual indicator for the amount of ammunition available. The feed system operates through a feed arm that is housed in the feed cover. Two feed pawls are linked to the front end of the arm by an intermediate link and move in opposite directions, moving the belt in two stages as the bolt moves back and forward during firing.
The MG 3 seen here in the stationary, heavy machine gun role, mounted on a stabilized Feldlafette tripod and fitted with an optical periscope sight.
The MG 3 has a quick-change, chrome-lined barrel with 4 right-hand grooves and a rifling twist rate of 1 in 305 mm (1:12 in). The barrel is integrated with the barrel breech. The barrel should be changed frequently during sustained firing. The gun is cocked and the barrel catch on the right of the barrel shroud is swung forward. The breech end of the hot barrel swings out and can be removed by elevating or twisting the gun. A fresh barrel is then inserted through the barrel catch and the muzzle bearing. When the catch is rotated back the barrel is locked and the machine gun can resume firing. Both the receiver housing and ventilated barrel casing are made from pressed sheet steel. A muzzle device is mounted at the end of the barrel and it acts as a flash suppressor, muzzle brake and recoil booster.
The machine gun is equipped with a synthetic polymer stock, a folding bipod and open-type iron sights (sliding rear U-notch on a ramp with range settings from 200 to 1,200 m). A flip-up anti-aircraft sight is also provided.
In a stationary, heavy machine gun role the MG 3 is mounted on a buffered tripod called a Feldlafette and outfitted with a periscope sight that can be used to engage indirect targets.
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