The 338wm beats the 300 wm in the bullet mass(frontal) which equals energy carried to the target. They are the same case necked differently with different pressures based on the diffrent neck sizes(this allwos more powder to poured into the 338wm). Marginal gains aside of course for each.

I push a 225gr out of 338wm at 3K...that's BA medicine for anything that walks NA soil, IMO. A 300wm with a 200gr may do similar but I'm getting a likin' to the extra frontal the .338 has! Both solid big game cartridges for NA.. no doubt.

Downside to both is recoil. Face it not everybody can handle the report of heavy for caliber reports from either of these out of full loads and a sporter rifle. What makes the 06's and 7mm rem mag so popular for those who are chasing everything short of the bear's who bite back!!!!