My brother-in-law gave me an old WW Greener 12 gauge shotgun with 2-9/16" chambers, and I am trying to modify a purchased 12 gauge die for this length. So I need some help from all of you machinists. I purchased a die for 3" 12 gauge shells I found on one of our favorite auction sites that fits my old Pacific DL-150 press. In measuring my existing 2-3/4" die and the 3" die I purchased, the crimping shoulder is 1/4" difference in depth as I suspected. So my plan is to cut off the difference between the 2-9/16" gun chamber, and the 3" die 7/16". I have on old Harbor Freight combination mill/lathe that my brother gave me years ago that I haven't done much lathe work with yet. It has a three-jaw chuck mounted on it that opens up far enough to mount the 1" diameter 12 gauge die piece in (which is about 3" long). So now comes the knowledge I am missing since I haven't run a lathe since high school metal shop class. It has a tool mount set up on it, and I think I can grind or mount purchased bits in it ok. So, if the die is rotating top down to me at a slow speed, where should I start cutting? Just below center a hair away from the 7/16" cutoff point? And after cutoff face the die end with a round or angled tool?