What 462 posted. I found a big fatty like that one a couple of years ago and it turned out to .69 cal. Bad enough to get hit by a .58, but that sucker would probably take your head off!
If It's like the one I've got, I'ts damn heavy. Maybe I ought to try and weigh it on my powder scale. Must have took a hell of a charge of BP to send that bad boy down range!