+1 to what Scortch said.
Also . . . . if it is the casing, is it brass or steel? If it's steel, it most likely is a "practice round" but you don't mention a projectile so I'm guessing it's brass?
Diameter and length would really help. . . . and perhaps a photo or two.
If a pair of '51 Navies were good enough for Billy Hickok, then a single Navy on my right hip is good enough for me . . . besides . . . I'm probably only half as good as he was anyways. Hiram's Rangers Badge #63