Your lead ball has the appropriate amount of lead-oxide "patina" on it to be from the 1800s or earlier.
SCDigginWithAK is correct, it is not a Minie-ball, which is a shape of bullet named for its inventor, French army Captain Minie, about 1850. Understandably, a lot of people confuse that name for a bullet with "mini ball" - but despite the confusing name, a Minie-ball is not ball-shaped. A Minie-ball's body is cylindrical with a more-or-less cone-shaped top (like modern-era bullets).
http://en.wikipedia.org/wiki/Minié_ball
As NOLA-Ken said, we need super-precise measurement of the ball's diameter, in 100ths-of-an-inch, to see whether or not its diameter matches up with any of the various known sizes of musketballs and artillery Case-Shot balls. Please use a Digital Caliper to make that measurement. See photo below.
I notice that your lead ball is somewhat out-of-round (meaning, not a perfect sphere), so there are three possibilities:
1- It is not a "projectile" ball -- as noted above, we need precise measurement to prove it is or isn't firearms-related.
2- It is a fired musketball which smacked into something with just enough force to slightly "smush" it out of its original perfect-sphere shape.
3- It is a civil war artillery shell antipersonnel ball, known as a Case-Shot ball. Unlike musketballs, which needed to be manufactured perfectly-round to load and fire properly in a gunbarrel, Case-Shot balls (which were contained inside an explosive artillery shell) did not HAVE to be perfectly-round.
I've included the possibility that it is a civil war artillery Case-Shot ball because you found it near Bristow VA... and there was a civil war battle there in October 1863 which involved artillery. At that time, the town's name was spelled "Bristoe."
Battle of Bristoe Station - Wikipedia, the free encyclopedia
Civil war Case-Shot artillery shells contained at least 40 (or more) lead antipersonnel balls. So, if your ball is from a Case-Shot shell, there ought to be quite a few more of them in the nearby vicinity. See photo below, which shows a sawed-in-half specimen of civil war Case-Shot cannonball and the lead balls inside it.