"... it looks like I'm going to have to look at the dies that will bump the shoulder without sizing the body. That's also a reason I can't just back off a f/l die." KN
KN, sizing down the body/web should not produce head separation's. That's usually due to stretching due to setting the shoulder to far back OR a springy action.
If you are setting you die according to directions, that being to "adjust down until the shell holder contacts the bottom of the sizer die", that's wrong!
That method does make ammo that will function and go bang but it frequently sets the shoulder back more than what is needed and that leads to case unnecessary case separations. Why so? Well, both our chambers and dies have tolerances in all dimensions. If we have a largish chamber and a smallish die, the case expands to the max but is resized to the min tolerances each time, even tho both may be within normal tolerances. If that's so, the cases are sized down and then stretched as much as .009" in each firing cycle. Solution? Adjust the die so YOUR cases will just chamber in YOUR rifle and no further, regardless of wheither it's rimmed, not rimmed or even a magnum with a belt, or if the headspace is large or small. In fact, "excessive" headspace shuold be of no real concern for a knowledgeable reloader.
It sounds like you have taken some careful measurements of your chambered ammo. I often see admonitions to adjust a sizer in "small increments" like a half or even quarter turn at a time; those are NOT small changes! Our dies have some coarse threads, 14 per inch, giving a change of 71.4 thousants of an inch per turn. Thus, only 1/8th of a turn covers the normal full range of headspace tolerance of .009", maximum to minimum. Only 1/16th of a turn puts us fully in the middle of that range and an adjustment of 1/64th of a turn changes the shoulder a bit over 1 thousant. It seems very few reloaders adjust so finely.
It is possbile to just turn the sizer down in very tiny increments until cases chamber snugly. But it's much easier to do it with a means of actually measureing what is happening to the shoulders. I like the RCBS Precison Case Mic for setting my FL sizers (and for constant seating depth adjustments too but I don't use the lousy "dummy" bullet seating device they include). Stoney Point/Hornady and Sinclair have good case measurement tools as well.
The often seen suggestion to "set shoulders back .001" or .002" is virtually impossible to do consistantly because of the varying springyness of brass. I try to get .002" on AVERAGE, with no shoulder set longer than the Case Mic shows on my fired brass.
My cases last for 6-8 loads with max charges, longer with light loads. My neck-sized cases for bolt rifle's fail due to neck splits, usually happening during the sizing/expanding step, and usually after about 10-12 loads unless they get annealed from time to time. Some of my neck annealed cases have been reloaded 20+ times! In fact, after more than 40 years of reloading and sevaral thousands of rounds, all with a fairly small stock of cases, I have NEVER had a head seperation nor a failure to chamber smoothly. I attribute that to the way I was trained to set my FL sizers. But I have no experience with break actions so your mileage may vary.
I wouldn't trade my Lee collet type neck sizers for any conventional neck sizers, including the button types.
