An "M" die is one made up primairly for lead bullets. Lyman makes them and possibly some other companies. It is an "extra" step that espands and flares the mouth of the case slightly so as not to shave lead when you seat the bullet. Keep the shape of the bullet is important to maintain consistancy and accuracy partioularly at 50 yards. Shaving lead will cause changing concentricity of the bullet and effect stabilty in my opinion.