Know this is an old topic, just a reminder.
Had a "already tried to fix it myself" job to do today. This is a through screw, holding a flat spring on the inside. Previous mechanic gouged at the slot with a screwdriver, pulling up a burr on the receiver, and then drilled an off-center hole in the screw. Fortunately, they gave up before they totally ruined the receiver and made it a even harder repair.
Took less than 5 minutes to do right. 4 1/2 minutes to find the left handed drill bits, get a drill, chuck it up, and 15 seconds to extract the screw.
The funny thing about this particular one, is that it is a THROUGH screw. It could have been done from the inside, drilling with a right handed bit. No danger to the receiver at all, and if it didn't thread out, once it was drilled out of the flat spring, it could have been tapped out of the receiver with no risk at all. Then, just clean up the threads in the spring and re-install.
With the left handed bit, just spun it out from the outside, I did that because they had already drilled a hole in the screw head. But, now have to peen back in the gouge on the receiver, or for a correct repair, tig it up and recut the screw recess.
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