AR15 chambers that I currently have and barrel lengths,
5.56; 11.5", 16"
223 Wylde; 18", 20", 24"
6.5 Grendel; 20", 24"
6.8 SPC II; 20", 24"
300 BO; 8.5"
The 300BO has it's purpose up too 300 yards and has been good and do not shoot sub ammo. 6.8 I would take everywhere! 6.5 I bought just because my first rifle was my Dad's 264wm (1961) and enjoy the 26cal! 5.56 I owned before the 223Wylde came out and has been a good light high volume gun. 223Wylde has been more precise over the 5.56. If there was a 6SPC then I would buy, because I would limit the bullets from 75-100gr. That would be like what Wilson did with the 300BO (larger case with up to 150gr bullet in the 300HAM'R). Just my choices anyway! BTW, no plans on buying the 300HAM'R.