Most provinces require 45 cal or bigger for big game and a 45 will kill deer for sure but is more than you need for small game. A 25cal is not practical and is the smallest I have ever known to be made - and it would be a custom barrel now = $$$. My experience with 32's convinced me they require too much special equipment like a 1/4" diameter ram rod and they foul up quicker than a larger bore. Think 36 or 40 cal: and most "Squirrel Gun" competitions are limited to 40 cal or smaller but 40 is a super compromise for convenience and power, and also can be super accurate. Mine likes 44 grains of FFFg and a .390 patched RB. With the 40 cal you can use a 3/8" ramrod which is hard to break, whereas the 36 cal requires a 5/16" ramrod. Also, 36 and 40 cal barrels will both likely be 13/16" across the flats so the larger bore makes for a lighter weight barrel and nicer overall balanced rifle. My experience.