Thinking about it - that bullet could have hit the ground 250 years ago, or last week. Without knowing a bit more, not sure how to know. You said you dug it out of an "old park" - not sure what you mean by "old" - has considerable to do with what caliber firearms were common or were in use in what era. So far as I know, was quite common for there to be one bullet weight used for any particular caliber - had to do with rate of twist and what was possible to be made at various times. Old time might have been black powder propellant at specific velocity - later might be smokeless propellant at much higher velocity. Generally, black powder loads tended to go with larger diameter bores - not always, but generally lots of fouling with black powder - whereas small bores became more possible and common with smokeless powder - not much powder fouling. I think I read that 22 Long Rifle sort of "bridged" that - was originally a black powder round, I think - then, circa 1930's became mostly smokeless powder round.