For years I used a hack saw to cut tubing. While watching "This Old House" their plumber used a small tool to cut tubing. The mini tubing/pipe cutter tool is designed to cut thin walled copper, brass and aluminum tubes and pipes. The cutter adjusts from 1/8 to 1-1/8 inch (3-28mm).
Mini tubing/pipe cutter
The mini cutter is my go to tool when cutting thin wall tubing and pipe.
Harbor Freight has one priced for the occasional user.