I think your 3-groove Minie bullet is more likely to be from during the civil war or afterward than before it. In the US, that version of .58-caliber Minie bullet was developed in the mid-1850s by the US Army and wasn't available for civilian use. After the war, many thousands of "war surplus" .58 US Springfield rifles were sold to the public, as big-game hunting rifles. Of course, bulletmolds were sold with them, so you could make your own ammunition.
I should also mention... although there may have been no civil war combat in your area of Maryland, soldiers were active everywhere. Frequent sources of civil war bullets and equipment are State Militia training-camps, which could be located hundreds of miles away from any battle. For example, people have dug Minie bullets, US Army belt-buckles, and bayonets at civil war training camps in Michigan and California.