This rifle features a coffee grinder in its stock to allow it’s wielder to grind coffee beans.

Edit:

one man in each cavalry company was to be issued one of these guns and he would be responsible for grinding coffee for the men in his unit while they were living out of the saddle. The real purpose, however, was a little different.

In January 1865, King’s invention was tested and reviewed by the inspection board. It’s in these reports that we learn the gun was actually to be used to grind grain, not coffee. Unfortunately, there were a variety of problems with this concept.

First, if soldiers were lucky enough to stumble upon a sizable amount of grain while out foraging, chances are that it would have been near a mill that could be used to grind the grain in a much more efficient manner.

The next issue was compatibility. The mill was designed to work with a solid buttstock, meaning it couldn’t be adapted for use in a Spencer carbine because the magazine tube was located where the grinder would need to go.

The final issue was weight. Officials believed that if a soldier was going to have to add weight to his kit, it should come in the form of ammo, not a novelty grinder.

Nonetheless, some did make it into production. Exact numbers produced are unknown, but it’s believed that less than 100 were made and only a dozen or so survive today. Even so, these guns turn up every few years, often enough to lead you to believe that far more than 12 survive. That’s because the parts laid around the St. Louis armory until the 1920s when an employee started installing them in original Sharps carbines. As such, it can be very hard to confirm if an example is original or fake.

TLDR: The newest theory is that they were meant for grain not coffee, and also probably didn’t work well.

https://www.thetruthaboutguns.com/coffee-mill-sharps-carbine-the-original-guns-and-coffee/