The original groove diameter from pre-war Ideal loading manuals states .407"-.408".  Modern info states .406" groove diameter.  We tend to trust old original info more. But, if you have one let us know what you've been shooting and what kind of accuracy your getting.  This bullet will also work in other cartridges such as 40-60 WIN, just let us know what diameter you need. We can go as small a .405"