Google is your friend:
SHOTGUN REQUIREMENTS
Any side-by-side or single shot shotgun typical of the period from approximately 1860 until 1899 with or without external hammers, having single or double triggers is allowed. Automatic ejectors are allowed on single shot break action, lever and pump action shotguns ONLY.
Side-by-side shotguns may not use automatic ejectors. Lever action, tubular feed, exposed hammer shotguns of the period are allowed, whether original or replicas.
The only slide action shotgun allowed is the Model 1897 Winchester shotgun, whether original or replica.
My understanding is Wild Bunch is a separate match in SASS shoots that allows period style 1911s but not M1919 BMGs.
on edit - I would really like to see match that did have 1919s though!