The math is fine and the equation is correct. What is confusing you is that I solved for the period per gun rather than for the rate. Rate would be (rounds/gun)/minunte. In my original the "per gun" part looks reversed, but it is not since the result units are really "rounds per gun" or rounds/gun. The units here are on a time/(rounds/guns) basis which is equivalent to (guns * time)/rounds. It looks odd until you reason through why the units fall out that way.