Those barrels are dead soft, and it's quite easy to cut them with a hacksaw.
I shortened many sporting 870's down to 18.5", 20" and 22" for guys who wanted to use their 870 as a HD shotgun.
My technique was to use V-blocks, hacksaw, square up on the disc sander, deburr, d&t for bead, fit the bead and give a quick hone to the area inside the muzzle where I installed to bead to smooth up the edges, and cold blue the exposed metal. Pretty basic, quick stuff.
The only down side IMHO is that you tend to end up with a short 870 that shoots high. The "bead with base" combination seemed to work best. If your high-vis front sight is taller than a simple bead you're probably fine.