Many years ago I built a couple of .25 caliber squirrel rifles - barrel by Bobby Hoyt on one, and Ed Rayl on the other one. Shallow (about 6 thou) round bottom rifling and about 48" twist.

Shot Hornady #4 buckshot .240" diameter with 17.5 grains of 3Fg powder. Sounds awfully scientific, but that's what a K-Hornet case holds, and it's a convenient powder measure.

No problem to put 3 shots touching at 25 yards - that's minute of squirrel eyeball. It was nice back then - I could see the sights!

The 20th shot with no cleaning between would go down the barrel as easily as the first, on either of those rifles. And that little 19 grain round ball sure did crack. Would go right through a squirrel's body at 40 yards.