I had a .32- rebarreled it to .36 and am happier with that one as it is a normal-for-nowadays barrel with wide grooves and slightly narrower lands.
The .32 had very narrow grooves that I didn't like. It also didn't group as tightly as I thought it should.
I have had a .40 & .45 and I know lots of guys love their .40's & .45's. There seems a big step up from .40 to .45, but not as much from .36 to .40. I
don't think I can quantify that, however those are my feelings on the calibre issue. If you shoot trail walks with deaf guys who wear ear muffs or
plugs, be ready to "lose" hits with all calibres from .25 to .40. At .45, there seems to be enough commotion from the steel so they are heard by all.
As a purely small game number, squirrels, bunnies and grouse, I would think anything from .25 to .40 would answer. As far as shooting anything in the
meat, even a .32 will blow a grouse apart. Stay away from the parts you want to eat.