Absolutely. People who are not proficient riflemen have NO business whatsoever with a magnum in their hands. Stepping up to a magnum is something you do AFTER you have mastered basic marksmanship skills and are able to handle the significant recoil and noise associated wtih shooting them.

Sorry, I know that was off-topic, but had to rant a bit...

Assuming that the bullet is of proper design for penetration on elk-sized game, I don't see why a 150gr. bullet wouldn't do the trick out of a .300 WSM. Folks take elk every year using 130 and 150 grain bullets out of their .270s... The problem is, that a 150 grain bullet designed for penetration on elk may have different ballistic characteristics and trajectory than a 150 grain bullet designed for proper expansion when used ondeer-sized game. As a result, you likely will end up having to resight your rifle anyway.