Here's my "Patented How To Cut A Shotgun Barrel With Minimal Tools" technique.
This is a "minimalist" method for when you don't have access to a lot of more sophisticated tooling.
First do the critical measuring and marking where the cut is to be made.
Wrap a piece of tape around the barrel OR use a small tubing cutter to lightly mark a line.
The intent is to provide a cutting guide line as square with the barrel as possible.
I don't recommend cutting the barrel with a tubing cutter, since this compresses or rolls the barrel inward at the cut, and you need to do a lot of filing to remove it.
Using a fine-tooth high-grade hacksaw blade in a good, high tension frame, make a light one or two stroke cut on the line.
Rotate the barrel and make another light cut. Continue until you have a shallow line all the way around the barrel.
Then just continue making one or twp light cuts, and rotating the barrel until it's cut through.
This makes an almost perfectly square cut.
Use fine-cut files and abrasive cloth to break the sharp outer edge of the cut and to smooth the front face of the muzzle.
Wrap abrasive cloth around the ball of your thumb and use that to break the sharp inner edge of the cut.
Touch up with cold blue.