There's the best way and then the cheapest way. Best way is with a metal lathe, but I assume you don't have one.
So, you can do what I did last year when cutting a 330mm barrel for my MP5SD, use a hacksaw and cut as straight a line as you can. Then put the other end of the barrel into an electric drill, and use a good cutting file to square the end, as well as round off the sharp edges. After that you can use a reamer (surprisingly the can opener on Swiss Army knives does well for this), countersinking bit, etc. to crown the inside edge (while still using the drill to spin it). Finish off the crown with fine and finer grit wet/dry sandpaper wrapped around a pencil.
Poor man's............ oops, sorry, more appropriately, the airsofter's metal lathe!
