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<blockquote data-quote="SDMF" data-source="post: 405139" data-attributes="member: 412">Fairly close approximation (rule of thumb) is 4:1 capacity vs. velocity.&nbsp; Meaning, for every 4% you gain in case capacity, you gain 1% in velocity.&nbsp; That is assuming both cases are loaded w/ the same projectile to the same pressure.&nbsp; I read the 4:1 in some book or post from John Barsness who (according to him) obtained that same rule of thumb from multiple ballisticians from multiple projectile and powder manufacturers as well as his own shooting w/chronographs and pressure test equipment.&nbsp; It is NOT something I came up with on my own.&nbsp; However, that rule of thumb bears out to be pretty darn close running pressure/velocity predictions in Quickload and it's reasonably easy to see reading the velocity specs on boxes of factory ammo wandering through he aisle.308Win:&nbsp; 56gn water capacity30-06:&nbsp; 68.2gn water capacity300Win:&nbsp; 92gn water capacity308 has 18% less capacity than 30-06 so should be ~4.5% slower308 has 40% less capacity than the 300Win so should be ~10% slowerThe curveball in all of this is loading manuals showing loads and velocities based upon different SAAMI max pressures for each cartridge.30-06 60K Max PSI308 62K Max PSI300Win 64K Max PSIThat 2K max PSI diff between the 30-06 and 308 is why &quot;on-paper&quot; the 308 always makes the 30-06 look like not much of a step up, and, if shooting factory ammo, they're going to be very close.</blockquote>

[QUOTE="SDMF, post: 405139, member: 412"] Fairly close approximation (rule of thumb) is 4:1 capacity vs. velocity. Meaning, for every 4% you gain in case capacity, you gain 1% in velocity. That is assuming both cases are loaded w/ the same projectile to the same pressure. I read the 4:1 in some book or post from John Barsness who (according to him) obtained that same rule of thumb from multiple ballisticians from multiple projectile and powder manufacturers as well as his own shooting w/chronographs and pressure test equipment. It is NOT something I came up with on my own. However, that rule of thumb bears out to be pretty darn close running pressure/velocity predictions in Quickload and it's reasonably easy to see reading the velocity specs on boxes of factory ammo wandering through he aisle. 308Win: 56gn water capacity 30-06: 68.2gn water capacity 300Win: 92gn water capacity 308 has 18% less capacity than 30-06 so should be ~4.5% slower 308 has 40% less capacity than the 300Win so should be ~10% slower The curveball in all of this is loading manuals showing loads and velocities based upon different SAAMI max pressures for each cartridge. 30-06 60K Max PSI 308 62K Max PSI 300Win 64K Max PSI That 2K max PSI diff between the 30-06 and 308 is why "on-paper" the 308 always makes the 30-06 look like not much of a step up, and, if shooting factory ammo, they're going to be very close. [/QUOTE]

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