Good Day!
I recently cut down a damaged Win 1300. It had the 28 inch barrel but has suffered a hunting accident (ran over) and the last few inches of barrel was damaged.
I cut a dowel to 18 1/2 inches and used it to gauge the correct length (I do not want to get to know the ATF that much) and then used a cut off blade.
Then I ordered a shotgun muzzle facing tool from
https://4drentals.com/
They are very helpful and have videos to get you going. You can even rent the tool for the choke set up of your choice.