Do you mean the one that goes through it horizontally, am i wrong in thinking that its a nut and bolt so you should be able to get something on the other side to stop the lot turning? If you mean the one that sticks up vertically and mounts to the turret, you might be able to wedge a screwdriver between the two parts of metal parts of the mount to prevent excess movement, but that ones a nylock nut anyway so wont be coming undone. Whenever I've fitted them in the past ive just done them up as tight as I could do comfortably.
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