Older Mini Thirties had the conventional .308 bore. Somewhere along the line they went to .311, which is the bore size of the AK47.
If you're plinking, hopefully it's .311. That way you can buy cheap import steel-cased stuff. If it's .308, ammo is going to be more expensive. The upshot is bullet selection will be way better.
Further research indicates the 0.003" difference doesn't matter with the exception of a small impact on accuracy. You should probably consult with Ruger though, just to make sure.
This post was edited on 10/4 at 9:14 am