Well, I really don't have any experience with anything other than .308. My dad had a 30-06 back in the day, but I never really used it. Since I'm already set up for .308 I'd stick with that. But if I had to pick a second caliber, and since we're talking bolt action rifles, I'd probably go with 338 Lapua. It would be fun to reach out a little farther! (if we were talking ALL rifle calibers, then I'd go with 6.8 SPC as my second caliber, but that's not in a bolt action rifle.)