Shoulder setback is caused by both the firing pin hitting the case and the primer igniting, driving the case forward. Under normal pressures, the case expands to fill the chamber and the shoulder returns to nominal. Low-pressure rounds won't expand the case enough and the shoulder stays back, creating excessive headspace. This is what happened to my .250-3000 cases; after a dozen or so cast bullet loadings, I went to full-power jacketed loads and there were several partial head separations upon firing. No harm done to the gun, but it could have been worse.
Dan
