Because Mark1985 (the slug's finder) sent me a PM asking for my input, I'm answering even though the slug has already been identified. The identification is correct. It is a 20th-Century rifled-shotgun slug. Some vaieties of that type are shown in the photos below, which were originally posted in this forum by BigCypressHunter about a year ago. Thanks, BCH. I saved them on my computer for answering ID-requests from relic-diggers who find these rifled-shotgun slugs, like Mark1985.