At first glance the gun looks to be some sort of auto-revolver. It is in fact a Steyr Mannlicher .222 Model SL rifle action and trigger group with some revolver parts tacked on. Note the double set trigger and Steyr’s iconic “butter knife” style bolt handle. It even retains the Steyr serial number.
Photo from a gunbroker auction.
Phil Steinschneider has a website detailing how be built a replica of the prop using a Steyr Mannlicher .222 Model SL action and a Charter Arms .44 Special Police Bulldog revolver.
Many thanks to Kevin for emailing me the Link.
UPDATE: Tam notes that this may still be classified as a firearm by the BATFE.