Here's a comparison to illustrate how the principles I've been trying to explain (above) work together in two comparable but different cartridges. The bullet (80-gr Sierra) and the barrel length (26 in.) are the same for both cartridges.
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<br>In the .220 Swift, a charge of 39.3 grains develops a peak pressure of about 65,250 lb/sq in. and drives the bullet out the muzzle at about 3,400 ft/sec (I say "about" because I'm rounding-off the pressure and velocity figures. And this charge really pushes the SAAMI maximum of 65,000 lb/sq in. for the Swift).
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<br>The .220 Howell requires a charge of 57.4 grains to develop a peak pressure of about 65,000 lb/sq in. and a velocity of about 3,760 ft/sec.
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<br>But by loading the .220 Howell with only 54.0 grains, I can get 3,500 ft/sec with a peak pressure no higher than 51,400 lb/sq in.
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<br>final comparison -- maximum Swift versus optimum Howell loads:
<br>powder charge - - - Swift 39.3 gr - - - Howell 54.0 gr
<br>velocity - - - Swift 3,400 ft/sec - - - Howell 3,500 ft/sec
<br>peak pressure - - - Swift 65,250 lb/sq in. - - - Howell 51,430 lb/sq in.
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<br>differences -- the .220 Howell
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<br>-- uses 14.7 grains more powder (instead of 18.1 gr more that it'd need to equal the SAAMI maximum for the Swift)
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<br>-- gets 100 ft/sec more velocity (instead of 360 ft/sec if loaded to SAAMI maximum for the Swift)
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<br>-- at 13,820 lb/sq in. less pressure (instead of the same barrel-burning pressure as the Swift)
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<br>With a pressure margin here of about 14,000 lb/sq in., you can see that I can increase the .220 Howell charge and get a little more velocity, and still be a good bit shy of the SAAMI maximum pressure.
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<br>But this would work against me more than for me. The increase in peak pressure would increase the rate of throat erosion, while the negligible flattening of the trajectory wouldn't be noticeable even at long range.
