Like me, others have complained of case neck splits with this cal with their reloads. And like me many use the Lee dies so I began with a letter to them. Truthfully I didn’t suspect the seating die was at fault until after reading their response telling me that bullets being seated slightly canted can be blamed on many other procedures. Studying the engineering of the product and comparing to the competitors products it became clear that with their floating centering device that must do so at the top and bottom simultaneously while not being supported anywhere, that this design solved a problem that doesn’t exist.
The solution came when I used the seating die I found in my 300 Savage die box, turned out to be a universal 30cal “KRUPP” American die with a truly floating, sliding and guided sleeve to center both the bullet and case mouth while seating.
Problem gone and my loads are perfect and this also explains why my 38Super loads had the same problem. I still like Lee loading products, just not the seating dies.
Hope what I have learned will be helpful.
Tom

Lee has done some wonderful things, IMO, but sometimes they drop the ball.

"Krupp American" and Herter's dies = one and the same.

I had case splits when I shot my 303 years ago but I always thought that was on account of the ancient ammo I was forced to use. For those of us less sophisticated, can you explain the floating centering device? Thanks for posting your results.

I seem unable to send photos to this site for some reason, this die even has a 1” cutaway midsection allowing you to have first engaged the case neck with bullet sitting in place. Then while you watch the travel upward you can see the fitted collet ride against the interior of the die, and when the tip of the bullet reaches the fixed cone at the top the sliding collet remains in control of the case neck and bullet to center both simultaneously. It works!

gnoahhh; Right you are, Herters 300Savage die set. I am thinking KRUPP is something brought back from Germany after the war.
This particular 30cal seating die could be used on any 30cal case as best as I can tell, it really works.