Without anything there to indicate size the first thing that comes to my mind is a .38 wad cutter. They are flat so that they will punch little round holes in paper and that makes the target easier to score. I do not think it is very old...d2
Can't even begin to speculate without a diameter. Nose is deformed so it may have been a target wadcutter . . . or not. It looks like a fairly modern pistol bullet and I'd agree with d2 (above) if it is .30" to .357" diameter.
The base looks like it may have at one time had a gas check (or at least the design would have allowed one to be added). That would definitely have been a modern/post WWII era bullet.