Good afternoon, looking for knowledge!!! I am very familiar with standard AR in 223/556. Looking for youth deer hunting upper here in Indiana. Public land not an issue, private only so I am not forced with straight wall. I just have extremely limited knowledge in the 6.5 Grendel, 6.8 SPC and 300 blackout. Anyway, I have standard AR lowers so it needs to fit. What calibers match to standard bolts? I see the term 6.8 SPC and 6.8 SPC II, and do not see an ammo difference with that. I am leaning away from 350, 450 and 458 for less recoil and true rifle cartridges advantages. Will standard uppers in the 6.6, 6.8 or 300 be capable of 2 inch groups at 100 yds?. Any other valuable info would be appreciated.