Sigpros,,,I had a 20" .223, match grade, barrel on an upper that Mike Mili made for me a few years ago in a 1-14 twist...The heaviest bullet weight that I could get it to shoot reliably (accuracy wise) was 55gr..
My initial purpose was to have a backup AR. to supplement my .204 AR, for shooting Prairie Dogs using 40-50gr bullets and that allowed me to push the 40gr close to the velocities of the .204 for a flatter shooting long range rifle...
When I went past the 350 yard distance, the bullet weight became somewhat self defeating as the rounds were more influenced by wind drift than the heavier (39&40gr) and smaller .204 rounds at slightly higher velocities..
I experimented with the 35gr V-Max as well with no noticeable difference between the 35 and 40gr V-Max in either trajectory or accuracy...