FL or NS for 300 rum & 7mm stw

[bigrack10]

I have a 300 rum & 7mm stw that I plan too use for long range hunting. I was checking too buy some dies and other components to up grade for better reloading, and a sinclair tech said I should always use FL dies on these magnum guns.

Is fl sizing better for magnum loads than neck sizing? If yes why?

Thanks for the help,
Randy