An Overview of the MG 42’s Roller-Locking Mechanism

 

An Overview of the MG 42’s Roller-Locking Mechanism

Brief Intro about MG 42:

                MG(Machine Gun 42) 

The MG 42 (Shortened from German: Maschinengewehr 42, or machine gun 42) is a German recoil operated general purpose machine gun used extensively by the Wehrmacht and the Waffen SS during the second half of the world war 2. Entering production in 1942, it was intended to supplement and replace the earlier MG 34, which was more expensive and took much longer to produce, but both weapons were produced until the end of world war 2. The MG 42 is designed to use 7.92 x 57mm Mauser rifle round and to be cheaper and easier to be highly reliable and easy to operate. It is most notable for its high cyclic rate for a gun using full power rifle cartridge, it averaged about 1200 rounds per minute, compare to around 900 rounds for his elder brother MG 34 and other US Machine Guns.

However, the MG 34 did have fundamental drawbacks, such as sensitivity to extreme weather conditions, dirt and mud, and comparatively complex and expensive production. Attempts to incrementally improve the basic MG 34 design is failed.

A draft specification was made and a contest was held for an MG 34 replacement, the German government ask to the companies who have no prior experience in weapon manufacturing. Three companies were asked in February 1937 to submit designs: Metall and Lackierwarenfabrik johannes GroBfuB AG of Dobeln, Rheinmetall-Borsig AG of Sommerda, and Stubgen AG of Erfurt. The design and mockup machine gun proposals were submitted in October 1937. GroBfuB AG’s entry proved to be the best design by far, emploiying a unique recoil-operated roller locking mechanism where as the two competing entries used a gas-actuated system. The GroBfuB company had no earlier experience in firearms manufacturing, specializing in pressed and stamped steel components such as sheet metal lanterns.

Dr. Eng. Werner Gruner one of the leading design engineers with GroBfuB, knew nothing about the machine guns when he was given the task of being involved with the project, although he specialized in the technology of mass production. Gurner would attend an army machine gunners course to familiarize himself with the utility and characteristics of such a weapon, also seeking input form soldiers. He then recycled an existing Mauser developed operating system and incorporated features from his experiences with army machine gunners and lessons learned during the early stage of war.

The initial trails of the GroBfuB functional model presented in April 1938 gave rise to improvement request by the machine gun contest board. The resulting GroBfuB MG 39 Prototype gun presented in February 1939 remained similar to the earlier MG 34, MG 42 overall the gun is more rugged and resistant to jamming than the precisely machined MG 34. Further trials resulted in selecting the GroBfuB MG 39 prototype gun for final production development. A limited run of about 1500 of further improved MG 39/41 pre mass production model guns, was completed in 1941 and by the end of 1941 tested in combat trails.

In early 1942 the final model of the weapon was officially accepted, and manufacturing of the mass-production model began, as the MG 42, contracts going to GroBfuB, Mauser-Werke, Gustloff-Werke, Steyr and others.`       

The Roller-Locked Recoil Operation:

The Roller-Locked bolt assembly consists of a bolt head, two rollers, a striker sleeve featuring a wedge-shaped front, firing pin, anti bounce bolt stabilizer, extractor and extractor spring, ejector and ejector bar, bolt body, bolt extension and roller for cams the feed arm.

           MG 42 Bolt Carrier Assembly

Figure A

The above figures shows the Before locked, Locked, Unlocked position of the Bolt head and barrel extension. From the figure A, the bolt carrier assembly is pushed by the recoil spring and the locking rollers are held in place in the undercut in bolt head before engaging with the barrel extension. The bolt will pick up the cartridge and try to chamber the round, simultaneously the locking rollers interacted with the arc shaped locking shoulders, the shoulders stop the rollers and it allows the bolt and the carrier go forward, simultaneously the Undercut that holds the locking rollers are matched with the locking shoulders. 

  Locking shoulder in the Barrel Extension

In this instant a small gap between the bolt head and the breech face, so the bolt try's to move forward under the spring pressure, simultaneously in this time the striker sleeve featuring  a wedge shaped portion interacted with the rollers mid portion, the sleeve push the roller outward, at this instant the lower profile of the holding undercut interact with the rollers lower diameter profile, also the rollers will follow the lower cam profile of the undercut hence the rollers will move from the matched Undercut to the Locking shoulders in a left and right direction by following the cam profile in the undercut and the stricker sleeve push the rollers into outside fully to engage with the locking shoulders, in this instant the rollers are placed in the angled surface of the locking shoulders. The reason behind the Undercut are in a camed path, if the under cuts are in a straight slot the bolt cannot move forward to fill the little bit last minute gap, also the bolt will not go to fully battery unless the undercut are camed downward relative to the locking shoulders, because the downward angle gives a path to bolt head move forward simultaneously pushing the rollers outward.

The MG 42 have a problem it was fixed in the late of the war, that is anti bolt bounce back, the bolt is not get into full battery after firing more number of rounds. To avoid these they use anti bolt bounce stablizer.

The "bolt stabilizer" term does somewhat describe what the device is supposed to do. It is more often called a "bolt catch". All it is, is a weight that can compress a little when pressed. It's job is to add pressure to the back of the roller wedge during that brief time that the rollers can extend outward and lock into the barrel locking piece. In effect, it causes the rollers to "catch" and prevent them from bouncing back out of the slots in the barrel locking piece and unlocking the bolt during firing. It was supposed by the Germans that the wedge may not have enough momentum by itself to keep the rollers from boucing out of the locking position. The shock absorber design of the bolt catch itself helps it maintain pressure on the roller wedge longer and without bouncing off the wedge itself like a ball bearing bouncing off an anvil. The bolt catch's own weight compresses it like a spring loaded weigh against the roller wedge for time necessary to ensure that the bolt locks and stays locked against the chamber of the barrel.

The above Figure Shows the rollers are hold by the undercut before locking

The above figure shows the bolt in locked position with out barrel extension the locking rollers are released from the under cut

 The above figure shows the rollers are engaged with the extension after locked

 Cut Session of the MG 42 Rollers are engaged with the barrel extension 

In unlocking phase, rearward force from the recoil of the cartridge ignition combined with the additional rearward force generated by the muzzle booster start to move the barrel and bolt assembly rearwards for a total distance of 21mm. These two parts start the unlocking sequence after the barrel and the bolt assembly have moved 7mm rearwards by the rollers higher diameter mid position following the unlocking cam in the trunnion of the MG 42. The unlocking cam forces the roller inwards and the locking roller follow the lower cam profile in the under cut of the bolt head and the roller comes into its unlocked position also reset the Stricker sleeve angled wedge by pushing the sleeve backward. By the kick given by the barrel the bolt assembly movers rearward and the extraction and ejection happens for feeding the new round.

 Unlocking Cam near the higher diameter.   of the roller (a curved surface)

The above figure shows the Unlocking Cam in the trunnion

                                         

          Figure A                     Figure B

  MG 42 Trunnion with unlocking cams

Figure A shows the barrel is forward before recoils, Figure B shows the movement of the barrel rearward after recoil. If you see the G3 rifle the rollers are placed in a round shaped surface in the trunnion and the rollers are placed in the angled surface of the Stricker sleeve, but here the rollers are placed in the angled surface of the Stricker sleeve also it was not placed a round shaped profile in the barrel extension, instead the rollers are placed almost straight profile in the barrel extension also it was placed in the curved profile in the lower cam profile of the undercut of the bolt head and it is not interacts with the curve profile in the barrel extension after locking, just interacts with the straight profile in the barrel extension and the straight profile in the upper under cut.