Jonah71,
My Papa had an old Remmy bolt that held like a bazillion .22 shorts. Seriously. The gun was as long as a Lee Enfield No3 mk1. My dad has it now, and has for many years, and that is what he used to hunt squills. It never had a scope, and you would hear the bullet hit the rat before you heard the gun go off. It was always a "PLOP ---- Blat" sound, and then a tree rat would fall.
I used a Glenfield .22 LR 9 shot. Like a Model 60 carbine with a fixed 4x. I get as many as dad would, but when I shot they would scatter for about 15 minutes. When he shot with that 10' barrel shooting shorts. Them squirrels would just pay zero attention to the noise. If we sat together he'd shoot all the close ones, and I'd shoot the ones that were off in a distance before we left to go home.
I would say a .22 short out of a bolt gun is about right for anything 15 lbs. and under at short distance. Distance being 50 yards and under. I shoot Super Colibri and CCI shorts out of my Heritage Rough Rider 6 1/2" SA revolver. They are quite, and take tree rats, snakes, rabbits, opossum, and the like with ease. I'd say the Super Colibri is about like a decent .22 cal pellet gun. I keep shots around 12-15 yards with them. Shorts out of the revolver are good to about 25-30 yards. I don't see the need to go larger than a .22 for smaller game. I not really judging anyone if it is legal in the state that someone is hunting in to use larger calibers. Heck. I would gladly take my .25-06 to the prairie to shoot ground squirrels. That I am thinking would be a fun day.