I'm gonna look to Dad on this one? He had a .32 Squirl Rifle he used, I have head of 30 cal as well? I never saw any or did he ever mention one in 22. but as a few here have said, no reason that they could not have? This size or caliber was popular for small game and my Dad took all sorts of that Mess up to racoons till I was about 10, if I remember right?
I think that probably .30 or .32 is the smallest practical size. Especially shooting real blackpowder. The fouling build-up would make repeated shots on a .22 cal somewhat trying. Also casting .22 cal balls or slugs is a bit of a chore. I've made 45 grain bullets for a .22 Hornet, and the rejection rate was pretty high, and I'm not even all that picky.
"Those who have long enjoyed privileges as we enjoy, forget, in time, that men have died to win them."
I had me a wall hanging replica of a flintlock years ago. Took off the pipe barrel and put on an old .303 British barrel threaded and breach plugged. Filed the frizzen and riveted on a pc of steel feeler gauge to make it spark. Drilled a touch hole in it and it fired a #4 sinker quite well. Took a few squirrels with it. Don't know what ever happened to it, but I wish I had it now for my grandson. Amazin' what 'Yankee Ingenuity' can come up with.