Firstly, where did you obtain Lapua brass in 300WM?
I have some (300) from the 1980’s, yet to lose a single case to splits or loose pockets in the 50 I use in F-Class Open.
My necks are skim cut to .0145” and I used a .336” -.335” bushing when I was using bushing dies.
Cheers.
Edit to add.
Just checked my notes on the Lapua brass, it measured .0145” when cut, but was around .016” prior to trimming. Bushing sizes are .332”-.333”, not as stated above.