Thanks for the newest photo, showing the casing's base-marking (called the "headstamp). Putting the photo into super-magnification, I seem to see "UMC" at the top, "50.70" at the bottom, and "US" stamped vertically on the left. Those markings mean your bullet-casing was manufactured by the Union Metallic Cartridge Company for use in the US .50-70 "Government" rifle, sometime between the late-1870s and 1888, when UMC merged with Remington Arms Company and the headstamps were changed to say REM-UMC.
The primer in your centerfire casing shows no firing-pin mark, so it is an unfired casing. The large amount of powder in the .50-caliber casing caused it to corrode to bits. The bullet that was in it should still be somewhere very close to where you dug the busted casing. Also, where you find one unfired bullet (or cartridge), there are often at least one or two more nearby. So, I'd go back and hunt the exact spot where you dug the unfired casing very carefully, to see if you can find the lead bullet from it (and possibly some more cartridges).