Dave, I agree with Cwbullet ...it is definitely a .54 or .577 Sharps bullet.
Although both rebs and yanks used Sharps bullets, the great majority were yankee-made.
To give folks accurate ID of civil war bullets, we need close-up photos and extra-precise measurements. (Lack of the latter is why we're having to guess about your Sharps bullet's caliber.)
Regards,
Pete [P.C. George]