The way I've seen it done is by making a tiny flat washer out of the thinnest shim stock you have, then placing it under the head of the pin and tightening it down, then verifying the position of the slot. If more is needed you go to the next thicker shim stock. Simple trial and error.

Sorta like shimming the hook on a double gun that's not worth paying to have it put back on face. Like my BSS duck gun.

May God bless America and those who defend her.