Accuracy Difference Between .308 and 30-06

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Originally Posted by Shug

Accuracy is consistency. Loads for most Garand-useful projectiles (150-180 grains) with mid-range powders come fairly close to filling a 308 case while they leave quite a bit of unused volume in a 30-06 case.

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Why is there so much unused case capacity in a 30-06 round? It seems odd considering that the 300 win mag exists. Is it due to pressure issues?