Given the shape of the cylinder notches, it's a black powder gun (and in remarkably condition, too).

The BP guns used notches that were basically triangles - nothing stops them from rotating back the other way. Part of the engineering change to accommodate the more powerful smokeless ammo was to add a rectangular notch to the triangular ramp, which holds everything together more securely.

Modern ammo won't cause it to explode, but it will rapidly hammer the gun and cause it to go out of time. Even though the peak pressure is low, the pressure wave comes on very quickly and they are not just not designed to handle that.

That's a really beautiful gun, and well over a century old. I wouldn't fire it at all.