MG42 full auto malfunction issue

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MG42 full auto malfunction issue

Postby CoryT » Wed May 16, 2012 3:17 pm

I've got an MG42 that I've just gotten a 7.62x51 conversion installed. Two barrels (new) bushing, flash hider, top cover and feed tray. The gun does run in 8mm, but I've got only a small lot of brass cased Yugo left, which is not totally reliable, with the ocassional light strike/dud.

The kit installes fine, just a slight clearance issue with the feed arm on the right side of the receiver track at the bolt rearward position, a little file work, no problem.

On test fire, a single round firing pops the barrel door open. This has never happened in 8mm. The bushing is correctly installed, pogo test is good, flash hider 1 click less than snug on the bushing, so a little rattle. Barrel door closes with normal slight thump, extension makes good contact on recoup plunger. Door latch has full engagement. Headspace is good, the fired case looks normal. Three different bolts and both barrels have the same issue.

Before the armorer manual arrives, anything I should look at? The only thing that comes to mind is the recoup system, but as stated it pogos cleanly, is correctly assembled and tight in the receiver.
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Re: MG42 full auto malfunction issue

Postby 42rocker » Wed May 16, 2012 4:21 pm

1st Is this a re-weld or?
2nd Ran fine with 8mm? Not in .308?

Ok, first thing that I would do is check the inside of the receiver and see about the fit and differences between the barrel extensions. I think that something might be keeping the barrel door from completely closing with the .308. Look and address that area. Measure the 8mm extension and compare to the .308 barrel extensions.

Good Luck.

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Re: MG42 full auto malfunction issue

Postby jbaum » Wed May 16, 2012 4:23 pm

You say you changed flash hider.... to what, the MG3 style? I always used the MG42 flash hider for shooting with .308 and never had a problem. Are you sure you have the barrel bushing in correctly?
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Re: MG42 full auto malfunction issue

Postby 42rocker » Wed May 16, 2012 4:44 pm

JBaum
Good Thoughts. On mine I replace the bushings and flash hider when I change to .308.

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Re: MG42 full auto malfunction issue

Postby CoryT » Wed May 16, 2012 11:04 pm

It's not a weld up, and it runs OK in 8mm. From what I can tell, the barrel extensions look the same, the contact on the recoup plunger is good, the door closes fully and there is no contact with the door and barrel until the door is open about 30 degrees. The latch and spring seem fine. The flash hider is an MG 3 type with no booster cone inside. I've not tried the MG42 booster/FH combination, not actually sure that will fit with the longer bbl bushing. Probably the FH alone will go on, so I'll give that a try in the AM.

It's just odd, if I try shooting a pair, just two rounds in the belt, I end up with the door open and barrel extension pokeing out and the 2nd round trying to chamber in a barrel that's not there. Since ejection of the empty is good, it seems the barrel pops out between ejection and feeding the next round. The door is straight and flat, the latch is good and the latch surface on the receiver is good. I can't see how the door opens without the latch moving forward and I don't see how it gets moved. It does not happen dry. Not sure if I can slow down video enough to catch it, but that's on the list for tomorrow.
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Re: MG42 full auto malfunction issue

Postby jbaum » Thu May 17, 2012 8:26 am

I use the MG42 barrel bushing, nozzle, and flash hider to shoot .308. They're not made to mix up some parts from the MG42 and some from the MG3.

Your barrel stop didn't come loose, did it? It's the block riveted to the receiver at about where the barrel change door is that stops the barrel from overtravel.
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Re: MG42 full auto malfunction issue

Postby CoryT » Thu May 17, 2012 9:34 am

Using the original MG42 booster cone and flash hider solves the problem, it runs fine with both barresl and all three bolts. I don't understand the mechanism of the failure to function with the other FH. The barrel stop is solid, and it's making good contact with the barrel extensionand it's not angled off. It's very odd. Given the absolute lack of any damage to the latching surfaces, it would appear as if the latch was somehow moving forward to the unlatched position and the same vibration pops the barrel off the recouperator plunger. There did not seem to be any excess recoil, and I'd actually expect to see back pressure on the barrel to be increased with the booster cone system in place. There is no contact between the barrel extension and the latch, though the clearance is very small. Interesting.

Well, in any event, the problem is solved, I just wish I understood WHY it's solved. Thanks, I'm not sure I'd have taken a chance on running the 42 FH and booster without confirmation it was acceptable.
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