Re: 338-378 load info
With Retumbo you should be able to start at 96 grains and work up from there. I am running 103 to 104 in my 338 Khan (338/378 improved) with 300 grain Bergers. 95 or 96 grains should be a safe starting load. I expect you should be able to work up to 100 to 102, maybe more since you have the factory freebore that Wby does.