Postby bmg17a1 » Sat May 15, 2010 11:28 pm
I would offer one additional piece of information to the well written description of the issue of out-of-battery ignition noted above in the beginning of this thread. Headspace of the MG42 type guns should allow a small gap between the breechface and the boltface on lockup of the bolthead in the barrel extension. This gap is on the order of .002-.004" and serves several functions. When the chambered cartridge is fully forward and the bolt is fully in battery the breechface should not impact the breechface too heavily, as the cartridge case absorbs some of the impact. An out-of-battery ignition is actually a consequnce of two mechanical actions: the result of the bolt recoiling slightly from impact with the breechface/case, and the oscillation of the rollers in the lockup channels. OTB ignition was a fairly common problem for the military throughout the use of the MG42 in combat. The Germans discovered through highspeed photography, that the rollers oscillate in and out during lockup, and could move as much as 1.5 mm inward from initial momentum towards full lockup. If primer ignition occurs during the inward movement of the rollers, the direction of unlocking of the bolthead, then the bolthead is not fully locked into the extension and the cartridge case can force the bolthead back on ignition resulting in a detonation.
In order to alleviate the potential for this combination of mechanical actions, and to avoid OTB ignition, the Germans altered the ignition timing of the primers for ammo specifically made for MG42s. Apparently this was successful since the bolt catches were not employed in 42s until after the war was over. Four different types of bolt catches were made, with the last model used with the MG3s.
Of course, special MG42 ammunition has not been available for many, many years and most shooters use standard rifle ammuniiton in the guns, whether in 7.92 or .308, so use of the bolt catch is mandatory.
Other factors can contribute to OTB ignition, one of which is broken or compromised recuperator springs, which will delay the return of the barrel to rest, with ignition occurring while the bolt is fully in the barrel extension but the extension is not fully forward insuring full lockup of the bolthead.
I have rebuilt over one hundred recuperators installing newly made springs, and found that most of the recuperator springs in the housing I've repaired were broken near one or both ends of one or more of the three captive springs, or the springs had taken a significant set and were quite a bit shorter than an unused spring and were no longer capable of exerting sufficient return energy to move the barrel swiftly back to rest while firing.
One of the design oddities of the MG42 is the recoiling barrel and the impact of the tips of the curved piece unlocking ramps against the inside of the extension around the breech. If one looks at the inside face of the extension at each corner it is possible to see small silver squares where the tips of the unlocking ramps impact the face. Under ideal mechanical circumstances the recuperator spring system would be designed to prevent the impact, absorbing the recoil of the barrel, and returning the barrel swiftly to rest, but never allowing the tips of the ramps to impact the inside face of the extension. Of course, such a sensitive balance is not possible for many reasons. Thus, the impact is part of the harshness of the recoil impulse of the 42, and if the springs are consistently to correct spec, the impact is minimized. Compromised recuperator springs allow the impact of the inside of the extension on the tips of the curved piece lockup ramps to increase as the springs deteriorate eventually to a point where the tips of the ramps crack and break off. The curved piece is a extremely hard alloy and thus quite brittle, and If this damage is not rectified, further damage to the curved piece and even to the recuperator plunger and housing can occur.
I have quite a collection of severely broken and damaged curved pieces from the breakdown of the recuperator springs.
Hope this helps.
Bob Naess
Black River Militaria CII