Have you ever seen the carnival version of the 1928 Thompson, or the 1919 browning .30 caliber machinegun? They are run off of compressed air and shoot small bird shot, (i’m not sure what size). There has got to be others here who could give you more details, but I know that they exist. Good luck!
Oh, here’s a thought…an HPA powered Gattling/Vulcan/minigun?
build a .177 that has a range of a mile and still penetrate 3 kevlar vests….wait a second, the military has one already…the .338 lapua. aw well I guess im just stuck with getting started on my first build.