After looking at the VLTOR receiver, I was surprised to see the entire top of the receiver has no rails bent in! Instead, everything behind the trunion is milled down flat, and machined rails are riveted into place.
I was very puzzled by this - I have assumed all along that the original PKM receiver was "just like" an AK, in that the rails are bent into the shell, not riveted in.
I was also under the assumption that reinforcement rails were then spot-welded to the inside so that the rails are double-thick.
Does anyone have any close-up pics of a military PKM receiver? I can't find much on the Interweb that has nice close-ups of the rails.
This is a good pic but it needs more light on the inside:
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