All I can tell you without a measurement on it is that having the reeded groove dates it after 1880. I'd say it's been in the ground quite a while to develop that patina. If you can get access to a digital caliper and get a measurement in 100th's of an inch that will tell us the caliber, without that there's not a lot more I can say.
I have some 38 S & W for my revolver that I just bought a few months ago. They are solid, with no visible groove. I thought it looked a little too small to be that, but I will look again and take a picture when I get home. Tough ammunition to find, but it still exists if you find the right place.
Looks like the cone on the newer bullets are smaller than the portion that fits into the casing... But the old bullet is is straight edged all the way down...