>>63858522
>>63858547
Can't cut it below 18.0"
Semis can't be cut shorter than 18.5" but that's not an issue with a side by side.

Aim to cut it for 18.375", if you cut short by accident it'll be 18.25" (still legal).
Measure very carefully and double check your measuring tape is correct. Mark the cut line with tape. Hacksaw slowly and carefully, finer tooth saw is better.
Take a fine flat file the width of the barrels and file the muzzles flat.
Take a round jewelers file and debur the inside of the crown.
Take 300 grit sandpaper and polish the muzzle and crown to a smooth finish using a twisting motion.
Install a new bead sight and plug the gap in the barrel ribs.

Obviously your new coach gun is going to be cylinder bore.
Enjoy!