Dragon6banger and Old Marine are on the right track... it appears to be a .45/70 rimfire cartridge, made for use in the M-1873 Springfield Breechloading Rifle (also called the .45/70 "Government" rifle), first issued in 1873. Being a rimfire, this is the early model of the .45/70 cartridge, dating from 1873 through a few years after that.
There's no danger for diggers about hitting an EXCAVATED cartridge from the 1800s-ERA with your digging-shovel. The "primer" has deteriorated. I would not say such a thing in a post if I was not 100%-certain about that information. In the 1880s, the US Ordnance Department conducted scientific testing on the Fulminate-of-Mercury primer to determine its "shelf-life" ...and the answer was 15 years or less.
But, please keep in mind, I'm talking about cartridges made during the 19TH-CENTURY that are EXCAVATED (meaning, dug out of the ground). Cartridges from the 20th-century can still have un-spoiled primer and powder in them.
The shot heard 'round the world. Or throughout the county, anyway. Wonder what the kick was like? Maybe one didn't want to survive the battle after 20 or 30 shots of this stuff.