Annihilator, SASS is not an historically accurate game. It actually relates more to the B Western movies of the '40's and '50's. Most folks try to be at least close to historically accurate, but nobody is gonna check your underwear to see if it is period correct.
Two other Cowboy Action Sanctioning groups are Western Action Shooters Association and National Coalition of Cowboy Shooters. They are both more demanding of historical accuracy. Neither are as popular as SASS.
When SASS started SXS shotguns were required. Then someone said, "What about the Winchester '97? It would be fun too." So they wrote a rule that said you could use it, but only load two at a time, just like the SXS shooters. Personally, I think they should be required to actually load two, as opposed to single loading and slam firing. But that's just me. I'm not gonna quit because I don't agree with all the rules.