Fullstock may be correct. If it is indeed a (squashed) civil war yankee .54 Burnside Carbine cartridge-casing, its "flared" lip has been neatly cut off. If it's not a Burnside, it may be a civil war US .50 Gallagher Carbine cartridge-casing. See the photos below. Note that the bottom of the Gallagher casing is more rounded than the Burnside's bottom.
Also, please note, the Gallagher casing bottom being distinctly wider than the Burnside's bottom, and to my eyes the bottom of ColtsPop's find looks wider (compare it with his finger's width), my vote is for Gallagher
Because it does look more than slightly "squashed"... ColtsPop, since you can hold it in your hand and compare it with these photos in real-life, please do so, and tell us whether you think it matches up with either one of these photos... or neither.