My LMG grip is a touch wobbly but the grip screw won't move in either direction. What's the trick to getting it out?
After removing internals,you should be able to drill the screw just enough to remove the peening. Then the screw will be more easy to tighten. Take a drill bit a bit larger then the screw,the idea is to drill just the tip to remove the peened part of screw. If you try to turn it without removing the peening,the threads in the nut might get damaged.
My theory is that the wood in the grip under the fastener head compresses a bit over time. Thats why all these "once tight" Type 81 grips become loose even tho the screw has not loosened.