Some shoters firmly believe that the reason the .243 Winchester tends to erode barrels more quickly than the 6mm Remington (which could be debated, and no doubt will be) is the .243's more sloping shoulder and short neck. The theory is that the shoulder tends to direct the hot gas into the throat just in front of the neck, speeding up erosion.
One piece of evidence in support of this theory is that Ackley Improving the .243 tends to slow erosion, but you'd have to shoot out a bunch of barrels to come to any firm conclusions.
IIRC McPherson used this as a reason behind his line of case designs. Something about keeping the turbulence inside the case neck through a combination of shoulder angle and neck length. I also recall reading that this is the reason the 6mm Rem doesn't suffer the same issue as the 243.
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