Well, let's rule some things out.
There are no rifling marks, so it was not shot down a rifled barrel.
There are no lubrication grooves, so it isn't a typical lead handgun or rifle bullet. All lead bullets have these grooves filled with a wax/bullet lube to prevent the lead from smearing the bore. It is tough to remove.
If you can put a micrometer or dial caliper on it to give some dimensions - that would be helpful.
Packrat1947