To today's installment: I've spoken with Lucilla Burns of the University of Cambridge's Fitzwilliam Museum, where the Persian spear butt from Deve Hüyük is held. She informs me that the object is hollow and very light.
I received my brass hame ball in the mail a week ago and it's eerie how similar this thing is to the Deve Hüyük spear butt. Seriously, this piece of horse tack looks more like an ancient artefact that it's not trying to resemble than a lot of "historical replicas" look like the things they are supposed to resemble.
Of course, it lacks the subtle decoration of the original, and is a shade larger, but on balance, I'd call these allowable drawbacks. It doesn't provide much counterweight, but that's as it should be, and it's still sturdy enough. Widely available and not too expensive as reenactment kit goes; just look for a two-inch ball with a sharply-defined socket. The socket on mine is one inch on the outside and 3/4 inch on the inside, meaning that fitting requires shaving off 1/8 inch all around the end of the handle for several inches, but it's flush with the shaft once that's done.