What type of barrel nut is this?

[palmettomoon]

Threads should be “standard” for an AR10/M5/etc. upper receiver.

They "look" 16 TPI even though you'd expect 18 with the rounded heel on the receiever. Have gauges coming Wed to be sure.

 

[palmettomoon]

Okay, received my thread gauge (how in Hell did I ever get on without this gem of a tool?) , in the mail today and confirmed 16 TPI and .209" at the tang so it's definitely a DPMS "pattern" LR308 upper. Now to find an 11" or 11.5" handguard. I was able to get a medium Riflespeed adjustable gasblock so going from the 13.5" to 11.5" gasblock will let me use that model.

Thanks for the help everyone!