Want to increase your carbon footprint by ten percent at a rate of about four hundred rounds a minute? Did I see some solutions for you!
Just in case the brass "Bulldog" Gatling from U.S. Armament didn't turn your crank, if you'll pardon the pun, there were some lovely Gardner guns in .45-70, too.
U.S. Armament is apparently also the company to which you should turn if you have a yen for an adorable breechloading 50mm Krupp mountain gun. And it's pre-1899, so no paperwork necessary, as long as you don't hanker after firing anything other than solid shot.
(The 50mm uses fixed ammunition, like your .22LR only much bigger, instead of a separate bagged powder charge and shell. I'm pretty sure that not even Old Western Scrounger has ammunition, so it would be a reloading-only proposition...)