The rim is a good fit, but there were a lot of other cartridges that had similar dimensions, including some handgun and rimfire rounds.
The only way to be truly sure would be to try to clean up the chamber.
Determining if it's rim or centerfire would also be helpful.
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
Baby Jesus cries when the fat redneck doesn't have military-grade firepower.