Can you set-back that barrel 2 or 3 turns if necessary? To clean up a sloppy machining-rechambering job?
But going back to your first question/s, I too would select the standard .257 Bob if not the 6mm Remington (standard), either with a 24-26in barrel! If you simply want to park yourself behind a shooting bench, maybe consider a heavy barrel?
But if you you're used to hoofing it a bit, maybe pick a walking-varmint contour
of some kind that you like?
If you don't like the case the 6mm and Roberts is based on (7X57 Mauser), then why not possibly consider the .25/06? Or better yet ...... a heavier-barreled .270Win for varmint blasting?