Here are a few hand made broaches I made about 50 years ago. This is the kind some of the early gun makers used. I probably got this idea from some old gun books back in the 50's or some blacksmithing books. These are not hard to make but take some time. I think I probably made these out of hay rake teeth.
there must be some relief stoned on the teeth. I have about 5 modern made ones I use now and they are longer and have a more gradual tapper. These old ones work just as well for squaring hammer holes. I just drill the hole a few thousands smaller than the diameter of the broach across the flats and then press these through with a socket on the back side. PS. The cherry is the cherry for the first complete long rifle I ever made. At that time round ball molds were few and far between for odd calibers. I made it out of a concrete chisel.
Moderator's note: Unfortunately the picture link in this post is no longer working.