http://www.rockislandauction.com/viewitem/aid/65/lid/1321

The Hotchkiss Revolving Cannon was developed in the 1870s as a competitor to the other manually-operated machine guns of the era, guns like the Gatling, Gardner, and Nordenfelt to name a few. What made the Hotchkiss stand out is that while the other guns were mostly built in rifle calibers, with larger options available, the smallest caliber Hotchkiss offered was 37mm. This was done because one of the primary applications for this sort of gun at the time was naval mounts to combat small torpedo boats. In that role, explosive projectiles were a major advantage, and international agreement restricted explosive projectiles to shells no smaller than 37mm.

The Hotchkiss has a cluster of 5 rotating barrels, but only a single bolt. It would chamber, fire, extract, and eject once with each full rotation of the hand crank, and it would hold the barrels stationary while the round was fired. The multiple barrels allowed a manual action to fire at a high rate, and also prevented barrels from overheating during extended firing.

This particular Hotchkiss is a modern-made reproduction, scaled down to use .50 BMG brass. However, the action of the Hotchkiss is not strong enough to use modern smokeless powder, so this can only be fired with black powder .50 BMG handloads.