There are alot of powders that will work very well. H-50BMG is just the best all around powder.
The very slow burning ball powders work great and will actually get a bit more velocity then H-50BMG. In some cases alot more but they are not nearly as stable with temp changes. If your shooting and hunting in the same area and can develope a load in the temps you will be hunting in, the ball powders are amazing in the 338 AM.
These ball powders are:
AA8700 (if you can find any left)
H-870 (again, if you can find it)
H-US869
Surplus powders:
WC872
WC860
All of these work very well. With the 300 gr SMK, I generally start at around 138 grains and work up from there watching for the regular pressure signs.
For stick powders, H-50BMG is by far the best but VV 20N29 will also work very well. It will also offer a bit higher load densities. I usually start at the same 138 gr with 20N29 and work up from there. In some rifles, you will run out of case capacity before you pressure out but still work up slowly and pay attention to pressure signs.