Small "O" ring sat in the thread relief cut, won't stop the mod sitting square on the shoulder.
But will provide a little bit of friction to prevent mod coming undone.
PS: Unless the shoulder is not square to the thread, in which case it will always be a problem.
Suggest us o ring and copper grease.
First day on the hinds yesterday, guys. 7 rounds fired (3 as a check zero first thing and then two hinds and two calves in one string) and the gun grease on the thread did the trick! No loosening at all and still had excellent purchase when I unscrewed the mod. Thank you all for the advice!