There are two different thread pitch options.
DPMS and SR-25 seem to match and AR-10 and Bushmaster (RRA?) also seem to match.
I offer this link as evidence:
www.jprifles.com/FreeFloatTubeSystem.htmlNote that they offer 2 different versions.
Best I can tell, the only difference is the thread pitch.
I have them both out in the shop right now, remind me to check thread pitch specs this weekend.
The thread diameter is identical.
Most of the handguard makers are making both barrel nuts available either currently, or will be soon.
It's easy to just make a nut with a different thread when the rest of the handguard system is the same.
I really wish every 308 was the same, it would just make it so much easier.
One other thing to beware of, Armalite's gas port is 3/16" farther forward on the barrel than DPMS so you have to be careful of handguard lengths.
Not sure where knights and Bushmaster/RRA fall on this as I have never had their uppers through my shop.