Generally, there is a belt or gear drive that rotates the entire upper part of the cannon. The rotation bearing which is secured into the upper fits on the outside of a hollow shaft or tube, through which the gas from the accumulator passes.
I know it's been awhile since someone asked, but are there any files available for printing out the rotating big gun cannons? What about different sizes, for lighter ships? If so......a link please.
Thanks for the reminder, this thread has languished for quite a while. I'll see about posting what I've got. So far just a twin 7/32" cannon. Not a final version with complete parts list, but it's almost final version and the 3d-printable parts are ready. I am also working on a quad 1/4" magazine, but that isn't quite ready yet either.