Sinarms has built me two 338 Edge barrels and I couldn't be happier. Devin is great to work with.
I've been following this pretty closely over on Savageshooters.
He's going to make the dies as well. So, starting with a 300RUM, all you'd have to do is FL size it in the die. That should make the 40 degree shoulder, there may be a little to fireform in the case wall, but it shouldn't be much.
All that would be left is to trim to length and turn the neck if it's too thick.
I'm going to use 338 RUM brass in my 338 SIN.