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Mine is a 3/32 Allen wrench, not a Torx. It is an older pistol, so they may have changed to a Torx. If so, I don't know what size it would be.
When you do get the proper wrench for this, the firing pin is spring loaded and may go flying across the room when you remove the screw. Aim it into a plastic bag to catch it. When you go to put it back in, tighten the screw until it stops, then back it off 1/4 turn to allow the firing pin to move. Degrease the threads beforehand and use blue Loctite (not red) on the screw.
Avoid dry firing the P-11. Excessive dry firing will cause the threads at the end of this screw to get beat up, possibly making it hard to remove.
buzzsaw