Yeah, I think it probably is the .45 Springfield carbine, which adds up based on the size and location of the find, etc. I struggled with a good way to photograph it, as it was squashed from two directions which didn't really flatten it, but sort of made a square-ish shape to it rather than a flattened or round shape, if you follow. The blunt nose helped, and location of the rings which I got a feel for by handling it. I did look at a lot of images on the web to compare. It may be a .36 but unless there is some way to weigh it, I guess I'll never know for sure.
Crazy, I searched the area again today and found a coyote of deer leg bone sticking straight out of the ground. That was enough for me. But hey, I moved on and found that musket ball.