It is legal to cut a barrel down as long as it is no shorter than 18". I would cut it no shorter than 18.5" to be on the safe side. Also if it ever gets the muzzle damaged you will have some extra barrel length to repair it.
You can order a 18.5" 835 barrel from Mossberg. You will have to call them to get one.
Other options are: The Mossberg 500 Tactical Breacher barrel and the LE 590A1 18.5" barrel will fit the 835 shotguns. These are 3" chamber barrels and you can only fire up 3" shells, no 3.5" shells.
These two companies have the exclusive on the Mossberg 500 Tactical Breacher barrels. No one else carries them.
Pro Patria "For Country"
Mossberg 500 Tactical breacher shotgun with one of my 7+1 tube ext that will also fit the 835 and 590A1 18.5" barrel shotguns.
Mossberg 590A1 18.5" barrel LE shotgun with one of my 7+1 tube ext