Niklas, you can safely assume that 90% of perfectly tight doubles will not have contact on all underlugs. Perhaps on new Brit or Euro guns but nearly every gun I've had a chance to actually stick a feeler gauge into showed space. OK, the really critical things are first the hook and pin then the rear bolting surface. You get these areas right and bobs your uncle. No one will ever know that any other surfaces aren't mated up perfectly. Absent fitting a new pin or adding metal to the hook the next best option is a piece of steel shimstock soldered to the hook. Absent the solder, Loctite. Even a piece of beer can will do in a pinch. The main thing is not to shoot a loose gun. That can stretch the frame or even crack it at the standing breech.
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