No single shot rifles allowed in factory class. Rifles must be fed from a magazine. Either box or tubular is fine. My old Remington model 510 is out.
Oddly enough, while a magazine of some type is required, you don't have to use it during the event. You can feed the gun one round at a time as you wish. In my case, I bought a single shot magazine for my 452 trainer and it fits the 457 perfectly.