I have used both 6.5 grendel Starline, and Starline Basic brass for 6 mm ARC. The 6.5 grendel brass is easy, just trim, lube, neck it down in full length 6 mm ARC die.
Got the Starline basic brass $125 for 250 pcs.
It's a lot of work...a retired machinist, I built resizing dies to neck it down in steps. You have to anneal before any sizing...than anneal after each time it goes through a different die. I annealed 1000 times for 250 pcs of brass. It turned out great and has the same water capacity after firing as the Starline 6.5 grendel brass.
Been doing alot of 6 mm ARC shooting with 55 grain to 115 gr DTAC. This brass will do 3 to 4 firings so far but when ya push "the book" envelope of AR loading pressure you ruined it.
I have 3, 6 mm ARC ar rifles, all 18" barrels. The Proof Research stainless are the ones to buy for accuracy. They will stabilize the 110 SMK wit a 7.5 twist. I prefer 108 grain and above for target shooting, the slight loss in velocity to the 105 doesn't make up for the better BC. With the exception of Berger 105 hybrid and 108 target BT are great Shooters. If ya had a lot of primers to waste you can make 6 mm arc out of 7.62x 39 brass cases, most have LR primer pockets....so you can use LR primers in your 6 mm ARC, if ya have em. May try some for curiosity to see what kind of accuracy to expect...have fun.