That Lapua .300 brass is discontinued stock if you can find any.
It was made by Norma for Lapua.
You can run .300 Win thru your .338 die with out the expander assembly to set the shoulders back to match your chamber .
Then trim case to -.025 less than the neck length of the chamber .
If you use Norma brass you will find a step where the base of the old .300 neck was .
This is because the necks are inside reamed durning the case forming process.
It helps to anneal the necks with a candel flame for about seven seconds-f.l. size again and neck turn.
I make 7mm Rem from .300 Win. to headspace on the shoulder for the first firing.
If you measure factory belted magnum rounds with a RCBS case mike the shoulder epands forward a LOT on the first firing in a SAAMI chamber.