Re: Advice wanted on my sizing / brass usage.
FL sizing every time is moving the brass much more than just neck sizing. I only neck size until I HAVE to FL size. Then when I do FL size, I set my dies up fresh every time and check to see how much I am bumping them back with a Stoney Point Headspace gauge. I like to only bump my shoulders back .002" or less. This will make it fit into the chamber easily, reduce work hardening, make it more accurate, and decrease the chance of making runout worse.
You may have a die that is really short and is cranking the shoulder back very drastically. Then you fireform it forward a mile every time you pull the trigger. Then you crank the shoulder back again and the whole process is like a tug of war match. Back and forth, back and forth until it snaps!
You could also have an unusually brittle batch of brass.
The only way to really see what is going on so that you can start the process of elimination is to get the headspace tool and see if you can fix the problem by changing or adjusting your dies.
My advice (besides getting the S.P. headspace gauge) is to get a Redding S type neck die with the correct bushing, and a Redding Body bump die with Redding COmpetition seater. This will give you the ultimate in control and adjustability.
If it's not far, it's boring.