Umarex MP40 bolt jamming cause found

The bolt jams happen most frequently in semi-auto mode. When they do, I find that an ever-so-slight pullback on the bolt resets everything, accompanied by a very faint "click". This has nothing to do with the often-mentioned feed problems associated with the magazine .... I have several magazines and no combination of mags or different ammo makes any difference. The problem is within the gun.

I disassembled everything using the following video ....
It's in German, which I do not speak, but the video is comprehensive enough that this didn't matter.

After pulling it all apart I found a small steel pin (3mm x 17.75mm) which had split in half. This pin is in the trigger mechanism and I'm guessing it takes the full impact of stopping the bolt. And after enough cycles it simply fatigued and split.
In the pics, half this pin is seen still in place in the left side housing and the other half is seen in place in the right half housing (I'm using a small screwdriver to point to the pin).

This is not a hardened dowel pin, a file attacks it quite easily.
I'll replace it with something more substantial.

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