I do not believe that any "normally" manufactured bullet will properly expand going that slow. Possibly one of the Hornady bullets designed for a tube fed 30-30. I believe that they make an 125gr lead nose HP. Possibly that.
I believe that you will have to go to a Lehigh Defense or the like. And this type of bullet can get pretty expensive. Another "theory" is a heavy/long bullet traveling right on the edge of sub/super that will destabilize apon impact,and tumble.
Please remember the ethical kill thing. --- SAWMAN