I shortened my .444 from 24" to 16" in the late 60's by hand, using an even easier variation of the second method above, which resulted in excellent accuracy & rifling protection.
You can do it too, with patience, a good eye, and the help of a hacksaw, files, either a small machinist's or carpenter's square, a double-ended can of valve grinding compound (fine/coarse), an electric hand drill, a 45-degree chamfering bit (common) and two or three large-headed, round head brass screws. [This is all local hardware store stuff - nothing exotic or expensive.]
Use the square to mark the barrel, all around; cut it to length with a hacksaw, file the rough cut smooth & as square to the bore as you can (checking from all sides with the square), slightly break the edge of the new muzzle/rifling with the 45-degree chamfer bit, chuck a round head screw into the drill, dip the screwhead into the coarse grinding compound and rotate it against the edge of the bore/rifling, keeping the drill going while you move the handle up/done/sideways/etc. Clean the muzzle & repeat with a new brass round head screw and the fine compound.
When you're done, push all the extra "junk" out of the bore/muzzle from the rear, clean & oil.
