Ditto...
I agree with these guys...you should be allowing at least .002" total clearance in your chamber...in other words, a loaded round should mic no bigger than .267 across the pressure ring of the bullet...
Additionally, and just as importantly, do you have the button screwed down tight in your Redding die? If so, another opportunity for bad Ju-Ju. Back the cap off 1/2-turn from locking down tight on the bushing. This will allow your bushing to "float"...and allow approximately the bottom .035" of the neck not to be sized. As your cases work harden, if you continue to stress that neck/shoulder junction where your neck-turning cut ends, you WILL continue to have separations. In 24 years, and in using sets of cases for dozens of firings, I've only ever had the problem you're describing ONE time...and that was with the first set of cases I used when I moved to the Redding Small Base PPC die, and all because I had the bushing locked down tight in the die, which caused the entire neck to be sized right down to the very junction of the shoulder. Allow it to float...that unsized .035" won't hurt a damned thing...