Small package critter getter?
How about a 22 K-Hornet, 218 Bee or 221 Fireball. Small case, powder efficient, and easy to load. I think anything bigger by case size would need a barrel longer then 12".
One other option you may want to consider in a 12" barrel would be either a 25-20 Winchester or 30 Reece. Again, small case, powder efficient and easy to load.
I would personally opt for the 30 Reece. Case capacity is similiar to a 32-20 Winchester so light loads would be easy to obtain along with high velocity stuff from Carson Reece.