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.