This is just a dumb question. The ground at Gettysburg is full of iron -- minis, balls, shot, and lots of musket parts.
My father used to dig around there when he was a teenager. He said one of the most interesting things he found were muskets with the barrels stuffed with five or six charges and balls. Apparently, during the heat of the battle, soldiers would load and fire so quickly that with the din and smoke, they couldn't tell if the thing went off or not. They just kept loading it until they finally figured out that the barrel was full, then threw the gun down, probably to head to the rear.
Digging has been banned there for years. There's no way they could excavate the entire battlefield.
You’re absolutely right that the area is positively filled with metal battle debris. Digging wasn’t always banned, and it’s not banned in adjacent areas annd tons of stuff has been excavated and placed on the market. Estimates are that there were 6-8 musket/rifle rounds discharged during the three day battle, plus artillery. I recently took my third trip there and bought a nice little collection of rounds + a fragment of an artillery shell. Going rate for a 54 caliber round is about $3.95
I had heard that before about the barrels. We drove from DC to Gettysburg to do the car tour & they said people in DC could hear the sound of battle. That’s 90 miles away. Not sure if I by that or not. Granted, they didn’t have noise pollution like we have today, but 90 miles?