That comma at the end of the address screwed it up...sorry. Just type in www.accuratepowder.com and go from there. .534 rim diameter is what it shows for the 300 and 338. My understanding is that the only difference in all for RUM offerings is the neck size and maybe a miniscule shoulder angle difference from caliber to caliber.
Trader338
The 300 and 338RUM are .534 I believe the others are the same. 300RUM case is 2.85" and the 338 is shortened to 2.76" the 338 Hart uses a 300RUM case and gives you the full length and 4 grains over the RUM. About another 100fps.