Re: 300-338LM Ackley Improved
E-mail the guys at Pacific Tool and gauge. In all likelyhood, they'll have something similar on file. Make sure you indicate that it is the 40 degree shoulder you're after.
They probablly won't have the cartridge dimentions per-se, but will have a Chamber reamer print with the reamer dimentions.
If they don't, they could easily design a reamer and make a honest .30-.338 Lapua Magnum AI.
If you are going to build one, or want to have one built, Hornady will make the dies off of a chamber reamer print from Pacific. You'll then be able to form your brass with these dies. It's a good way to go and would help cut down the waiting times required for custom requests.