Britt; It is a necked down 300 WM case;first pass with the form die forms the new shoulder and longer neck;second pass with the trim die brings the neck down to 7mm(no trimming required,then through the FL die to tighten up the neck.(Dober had to walk me through all this).
FFing the case blows it out to a straighter contour;the 300 Win Mag has more taper and a shorter neck.
The longer neck is part of the beauty of the Mashburn design, plus the straighter case,which gives a skosh more capacity. The longer neck helps if you build with a longer throat,as I did, because I can seat even 140 gr bullets close to the lands.No bullet I have used is seated into the case,except the 175, and that very little.
The 160 AB is seated to about 3.575 OAL;and the 175 Partition to about 3.515,due to the ogive mostly.
At the end of the day, I doubt there would be much practical difference between the Mashburn and the 300 WM necked down.
