Congratulations on your 338 RUM Sendero.
I have some pet loads for the 338 RUM but in the lighter weights, 200 gr, 210 gr, 225 gr and 250 gr.
Let me know if you decide to go with something lighter than the 300 gr MK's.
I have a load for the 225 AB that is not max but shoots .5moa in my 338RUM rem 700. It consists of the Fed215GM primer, the 225AB and 90 gr of IMR 7828 (listed max was 92 gr). OAL to function through the magazine. Should get about 2950 fps. I am going to try some Retumbo when I get done with bedding the new stock. You should be able to get around 3100fps with that bullet in your gun.
Leftys are the only ones in their right minds!
(and I ain't talkin' politics!)