I have an idea rolling around in my head for and ELR cartridge. I want to take a 300RUM and push the shoulder, neck, OAL, etc., back by .200". This would give you a case OAL of 2.650, a mini 300rum if you will. All the same specs except shortened by .200". I would also probably straighten the taper out to about 10 to 15 thousandths as well. It would yield a case with about 105gr capacity give or take and you would be able to seat the 230gr Bergers way out and still fit in a AICS or Accurate Mag. If my numbers are correct, it should be able to push those 230's up around 3000FPS out of a 28" bbl, making it better (drop wise) than a 338 Lapua moving a 300gr Berger Hybrid at 2850FPS, to a mile, and almost as good in the wind.
How would I go about moving the shoulder back that far? Is it even possible? My initial thought is to cut down a 300RUM FL Sizing die by .200" and just try it, but I think trying to move the shoulder that far will most likely result in a donut. Any help would be greatly appreciated.
Here are a couple of pics to help show what I am trying to do. The first is a 300RUM and the second is my shrunk down design.