Suggestions on the next Trail Gun?

Primary target wil be swine at the feeder so 15Y - 60Y shot. Otherwise walking-n-stalking or farm truck gun. I'd like a 7" barrel. It will wear a C-More Railway but would like something that may hold a FF III as well (gonna need a mount also).

My thought is 10mm or 321 GNR (.308). Can we squeeze a 321 in a Single Six?