If it were me I'd go 22br. Regardless of what you choose you'll need a new bolt/bolt head and new mags so imo might as well go for the 22br. 22ARC (just a standardized 22 Grendel variant, really) just like 6 ARC is really just a way of maximizing performance out of a small frame AR. Devoid of those restrictions, there isn't much reason to stick with any of those Grendel-type cases, not that there is anything wrong with them.
I'm personally a big fan of .223 (I've burnt out a few, they tend to go around 7500 rnds in my experience) because it's pretty easy to get 2900fps+ out of a 26" barrel with 75s (if loaded longer than AR mag length) and the 75 BTHPs are a great balance between performance and cost for my uses (practice for PRS/field matches 600 and in). If you keep your ammo to AR OAL you could also have one load of precision .223 that swapped between bolt gun and carbines (obviously that was in the before-times prior to the severe govt induced retardation we currently deal with). If you sub in the 75 ELD-Ms, you increase your performance considerably (and the powder charges between BTHP and ELD-Ms have been the same in my experience). Obviously you couldn't push 88s as fast in a .223 but they aren't particularly cost-effective for what I'm going for.
I'm personally a big fan of .223 (I've burnt out a few, they tend to go around 7500 rnds in my experience) because it's pretty easy to get 2900fps+ out of a 26" barrel with 75s (if loaded longer than AR mag length) and the 75 BTHPs are a great balance between performance and cost for my uses (practice for PRS/field matches 600 and in). If you keep your ammo to AR OAL you could also have one load of precision .223 that swapped between bolt gun and carbines (obviously that was in the before-times prior to the severe govt induced retardation we currently deal with). If you sub in the 75 ELD-Ms, you increase your performance considerably (and the powder charges between BTHP and ELD-Ms have been the same in my experience). Obviously you couldn't push 88s as fast in a .223 but they aren't particularly cost-effective for what I'm going for.
