You told us your bullet's length measurement but not its diameter -- which we need to know in order to estimate the bullet's caliber. Based on comparing its diameter with the US penny in the photo, it appears to be a "boat-tailed" copper-jacketed .50-caliber machine gun bullet, dating from the mid-20th-Century to the present. It also looks fired. If you clean the encrustation off the fattest part of your bullet's sides, you'll find rifling-grooves caused by firing it through a Rifled gunbarrel. Examine the photo below, in which you can clearly see the back end of the bullet is shaped like a boat's end, hence the term "boat-tailed.". That bullet was found by T-Net member Thrillathahunt. I saved the photo for purely Eductional purposes, like this reply.
