This is actually a DPMS LR308 platform rifle. It just seems that even though a lot of the peripheral parts are SUPPOSED to work for it, they're not all an exact fit. The example is the barrel nut from Fulton Armory. It's from their Titan line, which is based on DMPS, so it should fit, but it's a quarter inch too long for the delta assembly.
I ordered a DPMS vented float tube. It's a 16" Rainier-barreled rifle with a mid length system, so I'm just going to toss on a low profile gas block and drop that free float tube over it. The guy has a massive scope on the gun and doesn't care about iron sights. His sling is mounted on the A2 stock, so it's not like it's going to be inconvenient for him. It'll just be different than it was.