I have been fortunate enough to either measure myself or get detailed measurements of the four surviving Williams Guns. That is, I *think* there are only four ... some of these guns have moved around over the years but AFAIK here is where they are now:
- Kentucky Military History Museum (Frankfort)
- New Market Battlefield Museum (Virginia - this gun was at VMI in the past)
- West Point Museum (New York)
- Watervliet Arsenal (New York - either this one or the one at West Point was at the Smithsonian at one time)
All of these guns differ from each other in small regards, including the caliber (ranging from 1.4 to 1.57 inch). Ammunition is unclear, but was said to be a solid shot round and also a buckshot round, using fixed cartridges of self-consuming paper. I have seen dug projectiles that were ID'd as Williams Gun projectiles, but most of these were larger than 1.57-inch. The barrels are smoothbore, so the ammo would likely have been a spherical or perhaps somewhat elongated ball.
The gun worked on the principal of a fore-and-aft sliding breechblock, worked by a crank attached a spiral cam. This same crank worked another cam that operated a hammer on the side of the barrel that fired the standard musket percussion cap. There have been claims that the Williams Gun could fire 65 rounds a minute, but this would depend on the carefully orchestrated movements of the 3-man crew (especially the capper, who didn't have much time to place the small musket cap before the hammer fell).
The Williams Gun has been called a "machine-gun" (even the "first machine-gun used in battle"), but this is incorrect - it was not an automatic weapon. Each round had to be loaded and fired by hand.
If you can locate a copy, I published an article in a magazine called "The Kentucky Explorer" with more details on the gun (Williams was a Kentuckian and the most notable use of the Williams Guns was in Schoolfield's Ky. Btty.) - "Kentuckian Invented First Machine Gun Used in Combat During Civil War," The Kentucky Explorer 9(9), March 1995, pp. 54-55 (I didn't choose the article title - it WASN'T a machine gun ;).