I dislike having to say this, but they appear to be 20th-Century bullets made for Blackpowder Rifle shooters.
If possible, please precise diameter measurements (hundredths-of-an-inch) of the side-by-side pair, one of which is the one you're asking about. I cannot explain that one, unless it is the equivalent of "wadcutter" bullet for Blackpowder shooters.
Need to see "sideview" photos of the others shown in a row. The one all by itself above that row is definitely a "Rifled Shotgun" slug from the 20th-Century, still being made and sold today. See the photo below.