I had a 7 twist, 5R, .223 Wylde built thinking I would try the really heavy bullets. That has not panned out, but it shoots 53 Vmax, and 69/77 TMKs equally good so far. I would not be necessairly afraid of the 7 and it gives you the latitude to go bigger. One thing about trying to shoot both really long/heavy and short/light rounds is the OAL. You will want the longer throat for the 80s, but it will make your 55's jump a river.