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Bolts and Recoil Springs Question

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Hey everyone! New owner here of a late war gun (NC SWD).

I thought I'd turn to people with a lot more experience than me and ask this question:

I understand that an MG 42's rate of fire can be altered by changing out for a heavier or lighter bolt, and by changing out the recoil springs. Fair enough, but if I am looking at additional bolts and springs, what do I need to look at (or ask) to determine how a given bolt or spring will affect rate of fire? In other words, how do I know how a replacement bolt or spring will affect rate of fire short of weighing the bolt?

Looking around, there are apparently two common bolt weights (at least for the MG 3), 650g and 900g, with the lighter bolt allowing for the higher rate of fire. Is this correct?

My biggest worry is putting something in there that should NOT be in there, or will cause too much strain on the receiver or other parts, so just hoping for some guidance to keep it in good working order. Any guidance folks can offer would be greatly appreciated.



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The stiffness of the recoil spring in an MG42 operates within a short range and should not be altered. It has virtually no effect on rpm in any case. Rpm is directly affected by mass of the barrel and bolt and. Since barrels are standardized over all the verious iterations of the MG42 up to the present, bolts are the variable.WWII vintage boosters were issued with several different front orifices to increase boost but this adjustment barely affects rpm.

The early post-war MG42s made by Rheinmetall were contracted to the Italians who eventually wanted to reduce the rpm of their post-war guns which they believed would lengthen the service life of the guns. They were supplied with a very heavy bolt body and a specially consturcted buffer which reduced the rpm to around 700 plus or minus rpm. The heavy bolt created more destructive vibrations than the standard bolt actually decreasing the life of the receivers so they were discarded. This combination was revived with the MG74.

The very heavy bolts show up for sale now and then but there are virtually no buffers appropriate for use with this bolt in the US. One fellow made hydraulic buffers that were specd for the heavy bolts but he only made a few.

You are best served by using a standard bolt and buffer and standard recoil spring. This combination is tried and true. There can sometimes be minor headspace problems with mixing post-war Yugo M53 7.92 barrels with wartime bolts and this can happen more freuqently with mixing wartime bolts with post-war .308 Steyr barrels. There should be a few thousandths gap between the boltface and the breechface when the bolthead is inserted into the barrel extension and locked. It is easy to check. If it is too tired 😓 ght, the bolt head can become tightly locked in the extension.

Some bits of advice: make certain the bolt has a bolt catch installed in the body. Any of the available types are fine; use only proven, reliable ammo; use adequate lube on the the working parts and never run the action dry or without adequate lube.

The achilles heel of the MG42 is the barrel recuperator. This is a tubular part holding four stiff springs and is located under the left rail. It is designed to return the barrel the short distance from full recoil back to rest prior to the bolt returning to battery with a fresh round. Consider that the gun fires from 18 to 20 rounds per second, it becomes clear that the recuperator has a particularly important job. There is no reliable way to determine if the four springs in the recuperator are functioning properly until the gun starts to malfunction. malfunction of the recuperator will not damage the gun, but it cannot be left to deteriorate further without then causing damage. I can rebuild the recuperator with new springs if you want to have it done. To this point in time I have rebuilt about 100 recuperators and may or may not have rebuilt the unit in your gun. If you want to chat about this call me at 802-226-7204. Bob Naess

Edited by Black River Militaria CII
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​ I was actually hoping you'd see this and respond, so was glad to see that you did. You're working on my 08/15 for me, and I know you're considered an authority on the German MGs in the NFA community.

The information you took the time to provide is invaluable. Interesting that you mentioned the rebuilding of the recuperators. I bought mine from Tom Stewart, and he said the recuperator was fairly recently redone. I wonder if you did the work.

​ In any case, I'd love to give you a call and chat about it sometime. I know you stay pretty busy, so if there is a good time of the day or part of the week that is better to reach you, just let me know.

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