As Marauder clarified (since I can't seem to type in a logical manner), the no-exposed-hammer rule is for the slide-action shotguns...the only "pump" allowed is the 1897, with its exterior hammer.
As far as side-by-sides, anything (without an automatic ejector) goes. "Mule ear" exposed hammer shotguns are popular with the renactor crowd, if not with the "I wanna win" crowd. Stoeger Coach Guns are popular (I shoot one, for what that is worth). The gamey folks like the '97.
Winchester '87 and 1901 (lever action) shotguns are legal, if rare. I'd like to have one just for the cool factor
You cannot put a shellholder on the shotgun. All loading must be done from the person. My shop's standard "Cowboy Package" on side by sides includes highly polishing the chambers so you can "flip" the shotgun up and back to pop the hulls free while the off hand is bringing two new rounds up from the belt. Most new shotguns have rough chambers, and the hulls won't fall free. Same trick with the revolver...you should be able to hold the muzzle up with the loading gate open and have the cases just fall out.
Alex
"Lone Iguana"
SASS 21509