So this appears to be a lot smaller than a 9mil and it looks decently old. I have found CW Bullets and more modern bullets at this park site. Maybe someone will be able to ID it for me...CannonballGUY I'm looking at you! HAHA
just judging from the tape measure I'd say it's a .32 cal. 5/16ths is around .31 of an inch. An accurate measurement with a digital caliper would give us a positive caliber.... Agewise, hard to say, my gut tells me mid 20th Century, but that's just me feeling on it...
Stef45, sorry to disappoint, but my area of "deep" bullet-knowledge is the civil war era, with "some" additional knowledge of 1866-1890s US military bullets. Here's all I can tell you:
Your bullet is not a civil war bullet and not a military one. I agree with NOLA_Ken and Tony that it appears to be a .32-caliber, for pistol. In some of the photos it looks like it has some white "pure-lead" patina under the dirt, and if that is correct, I think its more likely to be from the late-1800s/early-1900s.
.32's were popular for a while as well were .25caliber pistols but are now considered "underpowered", now people carry .380 and 9mm and many argue the .380 is underpowered as well but no one has volunteered to get in front of mine.