Brass for both are easily made. Take 444 marlin case and run it up the FL sizer die. Out come said brass. No need to fireform. Load up full house loads and your in business.
As for the velocities in a 338 JDJ #2. My barrel is 14" and has an SSK Arrestor. So technically the rifled portion of the barrel is really 12.5-12.75"
200 grainers will do 2250-2300 fps 250 grainers will do 2000-2050 fps