A couple of my molds for hollow point conical bullets leave a little ring like that. It's from the hollow pin inside the mold. The base of the pin leaves a mold ring. Though that does kind of appear to be bi-metal, I think it's just an optical illusion caused by that ring. It does look more like a drilled hollow though. Maybe the drill chuck left a ring? Never really seen a straight hollow like that. Always used a tapered pin set myself.
ronlaughlin, any idea what the hardness was on those bullets? Seems they would need to be just a touch softer for whitetail.