Slug for a shotgun. 1970 +/- to current. Same principal as an airgun pellet or a badminton shuttlecock - weight forward for stability. They fit in a commercial plastic shot cup for reloading your own slugs.
Could it be a old style 410. shotgun slug? It seems to me some shotgun slugs had shapes like this in the past ?? but I very well could be wrong about that. A good accurate diameter of it could help in solving this.