Don't use a pipe cutter unless you are cutting pipe. It will 'crimp' the barrel, decreasing the diameter at the cut. If your 12 is in tough enough shape to turn into a 'home defense gun', use tape around the barrel and a fine tooth (32tpi) hacksaw, then using a file and a small square, true up the end of the barrel.
If you know someone with a lathe, it will be easier.
"When bad men combine, the good must associate; else they will fall one by one, an unpitied sacrifice in a contemptible struggle."
Edmund Burke - 1770