Both shoot a .30 caliber bullet. The only measurable difference between the two is velocity. Velocity matters for certain bullets to expand. I shoot Barnes Triple Shocks and I think the minimum velocity is around 180-2000 for full expansion. All this means is the 300WM will theoretically allow for a longer shot and still have the velocity for the bullet to perform. I like the 300WM as an elk round and it's served me well; ironically, I've never taken a shot at elk past 140 yards.
