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.html

Note 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.

Yes, the pitch is different on the DPMS and AR10. Also as noted above the gas system is
different. On a rifle length (24") set up, the DPMS rear face of the gas block is 5/16 shorter than the DPMS AR10. Bushmaster (RRA?) the thread pitch is the same as the AR10 but the receiver threads are 1/8" shorter so the barrel nuts will not work. The Knights are the same pitch as the AR10's but the thread pitch diameter is a little bigger (.015) so an AR10 barrel nut will not screw on there.
A note on the Bushy stuff, I have noted that on about half of the uppers I have set up with tubes
that the pitch diameter of the threads is hit and miss. Half or so of them are .025 smaller in pitch diameter so a standard nut will be real loose. I have made two different nuts to accomodate this.
The length one the Bushmaster tube is real close to the AR10 with the carbine being good and the rifle only about 1/16 long. The Knights newer guns are 1/2" shorter than the AR10 and the older ones are 3/8" shorter than the tubes on the AR10. Knights carbines are a whole different ball game, too


There you have it in a nut shell

Quoted: Also as noted above the gas system is different. On a rifle length (24") set up the DPMS rear face of the gas block is 5/16 shorter than the DPMS.

I think you mean to say that DPMS is 5/16" shorter than Armalite right?
I measured gas tube lengths and they are only 3/16" difference.
Seems like Armalite is allowing for 1/8" less carrier key engagement than DPMS.

Thanks for your insight, you certainly have seen more of these through your shop than I have.