I run 18" 7.5 twist in ARs and it stabilizes all the bullets from 58 gr to 115 gr DTAC.
So either twist rate 7.5 or 7 will stablize any 6 mm bullet available. I've shot 105, 107, 108, 110, 112, 115 gr bullets.
Thats the load data for 115 DTACs AR 15 6 mm ARC 7.5 twist Proof, it likes most of the 107s, 108, 110 and 115 are it's best bullets.
Remember between these two twist rates there is nothing to be concerned about...at all...more important is the quality of the barrel, the chambering job, and all the things that make a premium barrel accurate. The 6 ARC in a AR 15 will shoot alot of 1/4" 3 shot groups and most 5 shot groups around a half inch or less.
A close up shows how many times the bullet just enlarges the hole.
The 6 ARC is very accurate. This happens to be a Proof 7.5 twist...but a 7 twist with any premium brand will do.
No need for arguments.
Bore scope it., tapper lapp, get the burrs off the gas port, true reciever, polish feed ramps, center bolt carrier, glue in barrel, tighten barrel nut to 60 ft/ lbs, no more no less, although some start torque accuracy at 45 to 48, not me.
I mill a channel in the barrel nut if it doesn't align the gas tube at 60 ft/ lbs.
This is the important stuff, not the twist rate as long as its adequate for the heaviest, longest, bullet you intend to shoot at your altitude and lowest expected temperature...simple.