If you are going to use cracker shells, get yourself a starter pistol that shoots them. We have pretty much given up using live ammo and crackers in the same guns as for the reasons mentioned above. The pistols work fine, and I got to put the run on a polar bear last week using ours, we chase off dozens each year. Once he's running exploding the crackers high in the air keeps him running. By the way - bangers seem to work better than screamers.
Until bears grow feathers load up with slugs. There is almost no advantage to using shot - if the bear is close enough to be a threat the shot pattern won't have time to open up so the gun must be fired as accurately as a rifle and individual pellets have poor penetration. I said almost no advantage - the only time I load shot is if I'm in a populated area and I'm afraid that a slug might exit the bear and injure someone such as in a camp ground or town.
I don't know what part of the country you are in, but in Manitoba a loaded firearm cannot legally be carried on a quad. Walking, horseback, bicycle, or a canoe OK, but no loaded firearms on or in motorized vehicles including motorboats and quads. In a bear gun there is no limit as to how many rounds can be carried in the shotgun.
A fun drill is to tie a 5 gallon plastic oil pail to the back of the quad, and have the line extending past the shooter to the pail. The quad takes off, and you try to hit the jumping and rolling pail as often as you can before it gets to you. I set up the pail at 25 yards, and I can sometimes score 2 hits with my .375.