My M60 did the same thing, locking up during rapid fire practice. The flag was at least partially up when my gun locked up, I don't believe that there's a way to tell on a 642 without disassembly. Manipulating the hammer and lock eventually got it to free up again. I called Smith and they insisted that it HAD to be something other than the lock. Not wanting a gap on the left side of the trigger, I popped the sideplate and ground the nub off of the flag. It's worked like a charm ever since.

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"Shall not be infringed"