Lancaster - as with any ctg. you are making brass for, YOU must position the shoulder where it has to be.

With ALL rimmles ctgs. the shoulder postion is critical to the round's headspace. Due to this round's tiny shoulder, there are a LOT of fellows who should not be loading for it.

Yes - when expanding brass, the cases will shorten slightly. If properly formed, necked straight then necked down for proper headspace, the shortening is minimal, perhaps .020" at mose. You cannot simply neck them up and trust that the headspace is correct - it usually isn't. The brass MUST be made to fit the chamber.

There is no problem with using '06 or much better yet, .35 Whelen brass as long as you understand what is required. Neck them straight, then neck them down to properly postion the shoulder. The problem with '06 or Whelen brass comes from the junction of the original case's shoulder and neck. When necked up, it usually leaves a ring of thicker metal at that spot. That "RING" of thicker metal will be positioned in the middle of the neck of the 'new' case. It may or may not screw things up for chambering after a bullet is seated, expanding that ring, but will sometimes effect accuracy.

Starting off with new brass is always a good method, as I did with these .375/06IMPs. Posted to show the method of fitting, then fireforming. The new shoulder is formed to produce a CRUSH fit, virtually negative headspace, in the new chamber. Thus, the case is fireformed perfectly, with a perfect shoulder and no case stretch.