"I was wondering why a .338-06 would drive a 200 grain bullet so much faster than a .30-06 could. Anyone know? They have the same powder capacity."
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<br>No, they don't. The .30-06 holds more powder than the .338-06 when both are loaded to the base of the neck. When you neck any cartridge OUT (larger), the neck becomes longer (and the shoulder correspondingly shorter), reducing the net capacity of the body and shoulder.
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<br>The .25-06, OTOH, holds MORE powder than the .30-06 when both are loaded to the base of the neck. When you neck any cartridge DOWN (smaller), the neck becomes shorter (and the shoulder correspondingly longer), increasing the net capacity.
