IMHO, antler is too brittle to use as a wedge puller. You need something at least as strong as bronze, and better still, STEEL, for this kind of work. I Just used a heavy rubber mallet to knock a key out of a stock, but even then, It took a lot of pulling on the head of the key to get to to move out of the hanger. IMHO, that key is too tight, and the owner needs to use a needle nose file( jeweler's file) to open the slot in that barrel hanger.
It doesn't take much filing to get this done. Sometimes the problem is just a burr that needs to be removed. Sometimes the problem is that the wood of the forestock has either expanded or shrunk, and the keyway in the hanger was not given room for endplay, to allow for this movement in the wood. The result is the barrel key is binding in the hanger, or opposite side escutcheon, or both.