I am not typically a fan of steampunk, but will gladly make and exception for this little wonder. YouTube user Alex Smyth has created a steam punk gatling based off of a 10/22. The a steam only powers the rotary barrels (for ATF compliance reasons), but it reaches an impressive ROF with the hand crank wheels.
About the only thing he could have done better is to make it belt fed. You can see his other works and even purchase a few on etsy: https://www.etsy.com/shop/PhoenixSteamMachines