That is definitely a shot gun slug. Used to call them punkin balls. Diameter could vary due to caliber. All models of 410, 20, 16, 12 GA shot gun slug ammo was available. Most deer slugs now days a smaller caliber jacketed bullets in a sabot. These type were common up to sometime in the eighties if I remember right.
That looks like a rifled 12gauge deer slug for use in a smooth bore shotgun. The rifling imparts a spin to the slug so it flies true and accurate. A sabot is a smooth bullet like projectile that's in a plastic 2 piece sleeve that falls away once the projectile leaves the barrel. It's used in a rifled shotgun barrel. The rifling in the barrel makes the sabot spin, once it leaves the barrel, the sleeve falls away and only the bullet continues to the target. I bet you got a very strong signal when you passed over it.