Cutting a longer shotgun barrel to make it handier in tight places is NOT simply using a hacksaw to chop it off. Rarely have I ever seen this work out. If the muzzles are not even, you can cause side to side or even vertical stringing of the pattern.
Here I have marked the barrels to 18 1/4" and removed the access.
Using a belt grinder, I make the muzzles even. I use a square on the sides, but since there's a taper to double barrel shotguns I have to use a level across the muzzles to make them even.
If you leave the end open by the muzzles, debris and water can get inside the barrel ribs and cause rust. So I use silver solder to close the hole.
I strike it off with a file, and polish it with Emory cloth.
The chemicals used to color the end of the muzzles to match the rest of the gun.
Next it's off to the knee mill to drill and tap for a shotgun bead.
The barrel is now finished and ready to be put back on the action. It's going to be a little easier to handle in closer quarters, but reducing barrel length also reduces the effective range of the firearm.