Big ups to arfcom's midwestoffroad for figuring out the thread size on my friend's 308 win Competitor Corps pistol is M20x1.25 for the muzzle brake since we couldn't google the answer.

But now the new problem.   He was using factory  7.62 nato ammo today and on the second shot the breech won't open.  It turns about 3/4 of the way,  by then is stuck. We put a decent amount of force on it but didn't want to break anything.

Any guesses?   The other round shows no signs of over pressure.  He's shot it a few times at least before this,  but not this particular factory ammo.