Taken from the rule booklet:

Section 1.0   Single Shot Rifle

 

A single shot rifle is defined as a breech loading cartridge rifle without magazine or provision for additional cartridges, in which the action must be operated, and in which a cartridge or shell must be manually inserted for each shot. Bolt action rifles are specifically excluded regardless of age or model. The action may be of the falling block, tipping block, rolling block, tip up or any similar type whether made recently or in years past. Regardless of the action type, the rifle shall basically conform to the style and appearance of rifles made from the Civil War era to 1914 period either here in the U.S. or elsewhere. This will include both factory made rifles and rifles fabricated by custom gun makers of that period.

I believe the ASSRA Mission Statement, printed in each issue of the Journal in years past, mentions exactly what Froggie said. All except for the reference to the NMLRA of course!
Regards, Joe