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I had soo much fun in my last post "Is Sectional Density by itself worthless" that I just have to give it one more try. I've always been attracted to non main stream cartridges like the 257 Roberst, 6.5x55 swede, the 280 Remington, etc. Often these cartridges are not loaded to their full potential because of older actions (or Remington's screw up!) I will use the 30-06 as am example parent case: Cartridge SAAMI PSI 25-06 63,000 6.5-06 A Square 65,000 270 Win 65,000 280 Rem 60,000 30-06 60,000 338-06 A Square 63,000 35 Whelen 63,000 It makes sense to me that If Everything IS Held Constant (file, bullets construction, brass, etc.) than the chamber pressure should be the same. So for many years I have made the assumption that if you are shooting within a "Parent" case that all calibers should shoot a bullets with the same SD at the same velocity (Again this is assuming that everything but the diameter is the same across the bullets!) Part of my assumption is that sense SD is a mathematical representation of the bullets weight and diameter it will partly account for things like bearing surface and expansion ratio. So far this has worked out very well for me..or maybe I've just got lucky. I would like to hear constructive challenge of this theorem. And I'd like to thank 308nut for his healthy input into my last post. Mike