I think you new reloaders need to learn to anneal your brass on the necks and the shoulders to permit proper re-sizing. Please don't take this as an insult to your abilities. The 50BMG case has so much more brass in it that it retains a memory especially after work-hardening which occurs naturally on every firing. Annealing every time to every third time will allow for better control of your sizing operations.
I too size 3X rotating the case in the die about 120 degrees each time, I find it helps with concentricity. Being able to present the projectile correctly into the bore will help your accuracy potential. As we all know, it is the loose nut behind the gun that makes or breaks that accuracy potential...
Cutting a shell holder is a fix, and better than cutting the die which some have done. Another fix is trying a different lube which may have a different sizing pressure characteristic. Some use Imperial sizing wax which is very good, but I now use spray lanolin which I make for my own use. Dillon lube, Hornady One-Shot, and other spray lubes work also, and this application of lube prevents over-lubing which can cause shoulder dents.
Ask if you have other questions.