Here's a copy/paste of my reply on this in the What is It section :
As CannonballGuy said, it's a solid shot artillery shell, with tracer. Without a measurement, and just based on a comparison to your hand (assuming you have average sized hands) I would offer an educated guess that it's a WW2 U.S. 75mm shell. If so, it's most likely for the US M2 gun which was used in the M3 Lee tanks, or for the M3 gun which was used in Sherman tanks. I say that because those are the most common rounds likely to be found that would be the size of the one you have.
An accurate measurement as mentioned above, adding length and weight will help either confirm that, or start a new search for a match.
The paint is not original by the way, and if I had it I would definitely remove it
EDIT.... If you look closely at the driving band, (the part with the grooves) even though it's been fired there may be visible markings still on the high parts. The gun barrel may have wiped them clean but I have occasionally seen some still visible numbers on them. You may have to strip the paint to see anything, but if there are any partial numbers left those could help
I'll add to it.... This is most definitely NOT an explosive projectile, and the only way you could get injured with it would be to drop it on your foot or something. So no worries
