Don't waste your money on an extra barrel.. 24" is plenty long enough, espeically for a big hog of a shotgun like the mossy 835..
I have a 21" barrel on my Rem 1100 turkey gun, and I have made TRIPLES on ducks and geese with it..It would be even better suited for upland game, but the guns I shoot upland game with have 2 barrels..