5/8-24 needs 0.623" and that is barely there (IIRC 0.628 was the measurement) on the Tikka T3X Light 6.5 the OP has cut to 21". The smith that threaded it wanted a damned shoulder and insisted it needed to be done 1/2-28" because of that. Once a (3/4" long) shoulderless 5/8-24 adapter was screwed on, there was the same RCH shoulder as it would had been cut 5/8-24 direct. Damned much ado about nothing. Thread them 5/8-24 and throw a locknut behind it. But, Thunderbeasts says minimum 0.725" barrel. Exactly why is that much shoulder "required"? BS, it's not, but then, what would there be to post about...
I can't tell if you're being serious or sarcastic.
A suppressor needs a decent shoulder to stop against because that's what aligns it to the bore. Relying on a tight thread alone for alignment is not enough, especially if/when fouling builds up in the threads (mostly an issue when swapping between different thread lengths). A locknut is OK for a brake, not good enough for a suppressor IMO.
On a sporter weight 30 cal barrel I personally always choose and recommend 9/16"-24 rather than 1/2"-28 if possible. A larger thread is almost always better when it can be done right.
If a guy is having special adapters or collars made, you can have an adapter made to shoulder on the muzzle instead of a barrel shoulder; this lets you go with a larger thread size. I've done more than a couple barrels this way myself and machined adapters to match.
