I quickly learned to set my FL sizing dies to headspace on the shoulder, instead of the rim( i.e. to just "kiss" the shoulder), to guard against head separations. This means that as long as the cartridge will chamber, headspace isn't much of a concern. The chambers for rimmed/belted cartridges have often been long in the body, creating excess stretching from FL sizing/firing, which leads to case loss. Out of habit, I size rimless cases the same way. This is only a suggestion.
Mike
I chambered the rifle using 7x57 Mauser reamers and headspace gauges and cut the rim in a separate operation to clear the max rim thickness. My reloading dies are also std 7x57 Mauser dies.
