Ive always used rockset on my flashhiders but tonight, did not have any. I ended up applying a little bit of blue loctite to the back half of the threads on a new Ruger American 5.56 I pickup. Tightened it down with a crescent wrench and called it good to go. I figure the little 223 bolt gun will not generate much heat and shouldnt take much. For the loctite users out there. Are you applying a little or a lot?

I have used red loctite on mine for years.  Even HOT, I have never had one come off.  I use it pretty liberally.  It takes quite a bit of heat and torque to get the FH off if/when I remove it.

I do all mine with blue loctite and have never had one come loose without barrel blocks and a 3/4 in wrench.

1 or 2 drops should do it.  Most Loctites (purple, blue, red, black) have a working temperature range that goes up to about 300 degrees F.  There are also high temp versions of blue and red (246 & 272) that go up to 450 degrees F as well as ultra high temp versions of both (2422 & 2620) that go up to 650 degrees F.

I just torque mine a bit and call it good.  Never have had one ever back off.