I run collet die in several calibers including 260/308/300WM. Love them. Set up on a rcbs rock chucker to cam over.
You have something wrong or the collet is hanging up. You should take the die apart when you get it. Check for burs, clean, polish and lube it. Takes about 10 minutes to tear apart and make sure it’s working correctly.
The lee collet against my Redding bushing dies and a few others still produces some of the most concentric rounds
Only time I’ve crushed a case was when sizing immediately after annealing. The collet grabbed the neck early and crushed it it.
This is likely what is going on with your die/brass. Grabbing the neck early. The collet presses the “fingers” against a mandrel. The mandrel is one size and controls the neck size. Mine has always been .0015-.0002” neck tension with these dies.
It’s possible your necks on your brass are to thick coupled with heavier expansion in your chamber.
I always anneal, clean the inside of necks with brush, clean the outside of necks with fine steal wool, then size. Lapua or Nosler brass
Have used mine for thousands of rounds without issues since my crushed shoulder when I first used the dies. Still one of my favorite dies.
I use:
Lee collet/Redding body die
Or
Redding FL sizer with bushing
Take apart your die, clean, lube, check for burs, start with the bottom of collet touching shell holder with ram in upper position. Insert a clean piece of fired brass (mandrel must slide freely into neck) in shell holder. Run the brass in the die while bringing the die down 1/8 turn each time. Every time removing the brass and trying to slide a bullet in the neck by hand. When your close you’ll feel resistance in the neck. Start with caliper measurements on the outside of the case neck. Another 1/8 turn or so should be close. The brass will only size so far and you’ll feel the resistance on the ram.
My 308 runs .0015” tension. If I want more tension I’ll need to downsize the mandrel or order a custom one
I use no lube when sizing necks with collet