.223 Wylde, 1:8. I run mostly 75 Hornady, both HPBT and ELD-M(seated longer than mag). Lights out with these. I have also accurately fired the 55 V-max, only lighter bullet I've tried. Runs govt surplus brass standard FL sized with no issues. My barrel is whoever makes RRA for them. The Wylde chamber is designed to handle 5.56 NATO loads as well.
I have a friend who shoots a lot of coyotes every year with the 55 V-max and a 1:9 wylde. If you are favoring some lighter bullets for varmint use, and willing to stick to 68 or lighter for heavies, this would be a better option. His rifle does stabilize my 75 HPBT load, but may be marginal and we've never checked it at distance but 100 yard accuracy was there.