I would personally go with the 12 gauge, but that may be due to the fact that I have some 12 gauge rifled blanks. The 12 gauge will handle the recoil effects better and beat you up less, but it will also weigh more. If you would want to consider doing a 12 gauge with one side rifled, I can help you out on that.

You can either cut off only one side and sleeve a rifled barrel into it, or you could cut off both barrels and stub in two barrels, the only reason for doing this is because the monoblock will be easier to bore out if one barrel is not still attached. I also have some 12 gauge Beretta take off barrels that have various chokes in them, one of these could be used on the other side.

So, your choices are, a 12 gauge double with one side rifled, a 12 gauge with both sides rifled, a 12 gauge on one side, either rifled or not and a different rifle caliber on the other side. If you choose this last alternative you will have to stay with a fairly low powered rifle caliber because of the lock up on the Stevens 311,(not real strong). Yes I know they have been converted before.

The neat thing is that these rifled 12 gauge barrels came off of Savage guns, the same company that made your 311. Contact me if I can help you out. Bob