Boy, that would be a pretty long barrel. I have a Moss500 riotgun w/ 18" barrel and bought a 24" hunting barrel for the gun after the original purchase for dove/quail. The chokes work well with the longer unit and it's a vent rib, but I've never seen a barrel longer than 28" on a shotgun of any design. Do I understand you correctly?
You might try contacting Mossberg directly about the barrel and then have VangCompSystems port it for you. HTH,