Mounting a Lyman 48 on a Krag can be tricky so I called my friend John Wills because I know that he has mounted a couple. First the case is thin and the inside soft. I found this out with the Krag pieces I have as well. The problem with a 48 on the Krag is there are two holes and they are not in line so when you mount the receiver in the mill one hole will be on a slope. John uses a 3/6" carbide end mill and plunges down until he has a flat spot for his center drill. The other hole he uses a carbide center drill then a carbide drill, then tap normally.
