It's been over an hour and nobody else has yet posted a response, so I'll give some of my thoughts.
I've not shortened or cut a shotgun barrel previously. If I were to do so, I'd want the end finished or rounded relatively smoothly, then blued or painted somehow to reduce rusting. I'd also be concerned with drilling and tapping a hole in just the right place to install a bead. I also realize not everybody cares about how the barrel looks after it's been sawed shorter.
I'm fairly handy and do own a manual hacksaw, reciprocating saw, aka a Milwaukee "Sawzall", and other power saws (jigsaw and compact circular saw). Also, a drill press, spare beads with taps, tap handles, cutting oil, etc., but I realize not everyone is handy working with tools and guns.
The legal length without it being an "SBS" (aka short barreled shotgun) is 18" measured from the bolt-face with bolt closed/forward. Often, the barrel is at 18.5" to be extra safe, but some folks desire a 20" or 22" barrel for longer home-defense/tactical purposes.
I actually really like my 14" barreled SBS, but, to be legal, it'll take submitting a form to the ATF with a $200 payment; fingerprint cards; a photo; and a few months' wait for approval. I almost forgot the engraving of the receiver with one's name, city, and state in accordance with more ATF laws.
The easiest way would to buy a replacement barrel, making sure to get one of the right gauge, length, and barrel lug placement. For example, I understand most 870 barrels being sold have a relatively standard lug placement for 4-round magazine tubes but there were some 870s with either shorter or longer mag tubes.
Also, with a replacement barrel. The barrel may say OK for 2 3/4", 3", or 3 1/2" ammo, but make sure to use ammo no longer length than made for the subject 870's receiver and barrel for safety and functionality.
A shorter replacement barrel can cost another $200, and a replacement stock might cost $120. Some folks would rather just buy a new or used shotgun already in the desired barrel length or configuration. Good luck on the project.