My Thoughts.
Good dig, if the bullet is .44 caliber than it was probably for the Colt Army pistol. If it's .36 caliber then it was probably for the Colt Navy pistol (both were revolvers). They were both civil war period and were, of course, used post war as well. With no scale or measurements listed, it's kind of hard to tell.
I've dug a a large number of these bullets here in Virginia over the last 28 years of detecting. The bullets I've dug were both dropped and fired and it looks very consistant with the period Colt bullets. They're mostly found in Yankee calvary camps.
The killer or a quick identifier of post civil war bullets is the vertical striations found in the horizontal grooves. This usually means that it's post war. Don't see that here. Also, some folks think that just because a bullet has turned white from oxide that it's civil war period; not always so.
Also, don't let an "off the wall" location make a disbeliever out of you. Things can end up in crazy locations. A lot of Yanks brought their old Colts back North.
Like I said, if your measurements are close to .44 or .36 caliber, then I'd say it's a safe bet that you dug a bullet that was at least fired from a civil war period Colt.