Keep in mind that the bolt threads into the rear part of that muffler and also has the nut on it. So you tighten the parts with the bolt first. Get it tight, then hold that position with the screwdriver and tighten the nut up.

BTW, I was talking to a guy at a flyin last year who said that he stuck the bolt into the freezer to try and keep those things from loosening up in the air. They seem to have that tendency if you've loosened them for any reason. He actually had it backwards. He should have heated the bolt and and maybe frozen the muffler body, then tightened it all while the bolt was hot and the body was cold. You want it to shrink to tighten. The things we do................ Seems he'd lost a number of the parts in the air. (Locktite on the nut is a good idea.)