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We have settled on a 223AI and 6XC. We have used various other calibers, 17 hornet, 204 ruger, 223 REM, 220 swift, on up to 300WM for a shot or two.
Our 223AI's run a 53g vmax at 3500fps and top out at 3650-3700fps when the bullets self destruct. 28" 8 twist 6 groove barrels. This is with longer freebore, setup for AICS pattern magazines. Starline brass is absolutely fabulous for this, I've built or rebarreled 8 or 10 barrels, and have yet to get one that doesn't easily shoot under a quarter minute.