There is nothing mysterious or difficult about cutting a shotgun barrel.
The easiest way of course, is to use a lathe and lathe dogs, with the bore supported between centers. Advance crossfeed to cut off barrel, deburr, and you are done!
The home gunsmith or hobbiest will find it easier to simply hacksaw off the barrel using a fine-tooth blade, and finish off the end with a mill file. Check for squareness, deburr and you are done. A dab of cold blue will hide the fresh cut steel end.