Looks like an old 40mm antiaircraft gun. Most current models you see in photos are multi-barrel jobs, but single barrel ones were very common years ago. A gravity fed magazine would be attached to the top of the breech at the rear and the rounds would feed by gravity.
Here's a US Bofors model.
"INSIDE EVERY PROGRESSIVE IS A TOTALITARIAN SCREAMING TO GET OUT" - David Horowitz
That's a Russian 61-K or the Chinese copy Norinco 55. 37mm. Actually a slightly changed replica of Bofors 40mm. Why US? Bofors was Swedish, technology by Krupp.
"You know, Paul, Reagan proved that deficits don't matter." Dick Cheney
C'mon folks, thats WWII vintage armament but with a long history since the Finns still had them in active service as late as 1999! Be the first in your neighborhood to a acquire your very own, just for $9,000! We are talking museum-pieces here that can be put to the test in a pinch!
"Only two things are infinite, the universe and human stupidity, and I am not sure about the former."
Well shit...now if they could only afford washing machines for the students...