It is absolutely a modern-era bullet, made for use by Blackpowder Muzzleloader rifle shooters. It is called a "Maxi-Ball" by its manufacturer, as shown in the photo below. You can buy them pre-made by that manufacturer, or buy a bulletmold from Lyman for casting your own (see the second photo, from an online catalog of Lyman molds).
I'll add a third photo, showing some various forms of Maxi-Ball bullets that come with a bore lubricant (called "bore butter") in their body-grooves.
I should mention, Maxi-Ball bullets are made in more than one caliber (as the third photo shows). I should also mention, Maxi-Ball bullets have existed since approximately the 1960s, so the ones we dig have had plenty of time to get some greyish/white-ish lead-patina on them.
I hope this post's information and the photos provide enough proof to forever settle any debate about what that version of bullet actually is, and the time-period they are from.