A lot of info here. I tested H322, XBR, and AA2200 in my 6X6.8. I settled on 30.9 XBR, 41 primer, 58Vmax in Hornady brass. I get 3371 fps and no brass beatings. The gun shot well with all 3 powders and can make a lot more speed but was beating up brass. Adjustable gas block helped but when I went suppressed I can't dump enough gas to get back where I need to be. I probably need to move the AGB and block some of the gas hole for more control, but the gun does what I need it to do. I was shocked at the difference between the 6X6.8 and a .223 on coyotes. I did make note in my testing that 41 primers made about 50 fps more than 400's and 450's made about 20 fps more than 41's across the board. Also found a big difference in max loading between Hornady brass and old stock SSA. Never measured the capacity but there is a difference so if you load the wagon and change brass back off, or you'll be auto ejecting primers!