A shame I can't edit my past posts… oh well.
Here is another locking mechanism that might have merit for an OS rifle build - flapper locks. Shamelessly ripped from a forum I found.
"Here is an idea I came up with and I think it allows for linear movement locking and prevents out-of-battery firing. (I think)
The idea here is that the flaps are simply pins that extend or retract with the bolt carrier. It requires a long travel gas piston (or it could be a straight pull bolt or pump action). The locking lugs pivot on small pins, but the pins don't carry any load. The lugs transfer the load directly from the bolt to the receiver.
This seems like it could be build from blocks and plates of steel. But I know that it is only a beginning point.
It is set up square, because one version of my design uses formed plates of sheet metal for the receiver. I think this could be done in a round receiver. But it would need part of the receiver to be squared to align everything and provide locking space for the flaps."