When the production of MG34 models was obviously insufficient for the German Army’s needs, a new design of weapon was begun, one which would be easier to mass-produce. For the first time the design was not left entirely in the hands of gunmakers; a Doktor Grunow of the Johannes Grossfuss Metall und Lackierwarenfabrik of Döbeln, experts in pressing and stamping metal, was called in to advise at an early stage in design. As a result the final design was specifically laid out to suit stamping and pressing processes, with welding and riveting used for assembly.
The action of the MG42 was modified to use a non-rotating bolt locking into a barrel extension by two rollers caromed outwards. Unless the rollers were out, locking the breech, the firing pin could not pass through the centre of the bolt. On recoil the barrel, bolt and barrel extension recoiled locked together until cam tracks in the gun body moved the rollers inwards to release the bolt.
Movement of the bolt drove a feed arm mounted in the top cover of the gun which in turn operated paws to feed the ammunition belt; the design was most ingenious and fed the belt smoothly and reliably and has been widely copied in other weapons since its original appearance in the MG42...(see more at: http://ww2-weapons.com/)