I feel like the 6 arc would get about as close as the 6.5 Grendel, non commercial?
The 6 ARC is a "commercial" round - it has cleared SAAMI standardization and has big brand factory ammo and rifles available.
The 6mm Max isn't a 30rem/6.8SPC derived case, it's a 350 Legend case necked down to 6mm pills and trimmed to fit 100grn (not necessarily 105/108) bullets into 2.3" mag length.
There do exist several 6mm variants on the 6.8spc case, but none are commercialized. The 24 Nosler is probably the closest to commercialization, it's a 22 Nosler trimmed back and necked up to 6mm - so it's a .378" bolt face like 223/5.56, but uses the 6.8 SPC case body... The 6 Hagar has been around for a long time, as have several other 6-6.8's, but they have the problem of over-length cases so when restricted to AR mag length, they can't use the long, high BC bullets we all prefer to use in 6mm rounds.
Therein really lies the rub - the 6.8 spc case is just too skinny to fit enough powder to do what it needs to do when restricted to 2.3" COAL with long, high BC 6mm bullets. Even the standard 6.5 Grendel case is really too long for many of the higher BC 6mm bullets, so if you push the shoulder back on the 6.8spc case to fit the same high BC bullets, you end up with even LESS case capacity due to the smaller diameter, and performance wanes even worse. That's why so many of us were shooting 6mmAR's/243LBC's/Turbo40's/Grinches/Rats/FatRats/etc based on the Grendel case before the 6 ARC cartridge was standardized, and NOT shooting the 6.8SPC based wildcats.
