The Hotchkiss Revolving Cannon was designed by an American, Benjamin Hotchkiss, in response to the Franco-Prussian War. Hotchkiss chose to use large caliber shells in the gun instead of rifle cartridges (like the American Gatling or French Mitrailleuse) for greater anti-personnel effectiveness. The Congress of St Petersburg prohibited exploding ordnance with projectile weights under 14oz (because exploding bullets were deemed inhumane), so Hotchkiss based his design on a 16oz (37mm) cartridge.
The mechanical design is fairly simple – the gun uses a single breechblock and firing mechanism, to which the barrel rotate one at a time. Each complete turn of the crank handle completes a firing cycle, with one round loaded, one fired, and one extracted. The mechanism is geared such that the crank handle us turned continuously, but the barrels are stopped and fixed during the actual firing process (this improves accuracy and can reduce the likelihood of jamming). Ammunition was fed via 10-round clips loaded by an assistant gunner.
During testing by the US Ordnance Board in 1876 and 1877, a total of 1,136 rounds were fired from a 37mm (1.5″) gun with only 5 malfunctions of any kind. One projectile failed to explode on impact, which was found be to the result of a broken firing in in the fuse. The other four malfunctions were light strikes early in the testing – the firing pin spring in the cannon was found to be faulty and replaced. After that repair, no more problems were recorded. That is an impressive performance for a repeating cannon in 1876, and the Ordnance officers were rightfully impressed. They were also impressed with the manufacturing quality and mechanical design of the gun – particularly how the barrels are set in place and not rotating while loading and firing occurs (unlike the Gattling gun and other contemporaries).
Primary use of the Hotchkiss revolving cannon was as navel armament, to defend large ships against smaller and much more agile torpedo boats. In this role, the Hotchkiss competed with other mechanical machine guns like the Nordenfelt and Gardner, but it was of a heavier caliber than the others. In addition to its original 37mm chambering, Hotchkiss cannons were also made in 47mm and 53mm in response to military demand. They were adopted by a number of nations including Brazil, France, China, the US, Holland, Greece, Chile, Argentina, Russia and Denmark.
This Hotchkiss Revolving Cannon was captured from the ruins of the Spanish cruiser Vizcaya after the Battle of Santiago Bay in 1898. It was returned to the US by Captain Caspar Goodrich (USN) and put on display on a Gattling carriage at the Connecticut capitol building in Hartford (click here to download photos at high resolution).
You can find a gallery of several types of Hotchkiss revolving cannon and description of the different variants (as well as other early mechanical guns) at Victorian Ship Models.
We also have a testing report from US tests conducted at Sandy Hook New York in 1876 and 1877, which includes translated excerpts from French and Brazilian testing as well: