Interesting. It's certainly true that under both SAAMI (US) and CIP (European) standards, the maximum average pressure (MAP) for 12ga is lower than the 20ga MAP. But looking at the Alliant Powder website and comparing 1 oz loads, I can find a 12ga load with a peak pressure of 10,870 psi and a 20ga load with a peak pressure of 10,300 psi. Depends on the components used in both loads.
How does one go about determining the total pressure for a given load? When comparing MAP, while the 12ga standard is lower than the 20ga, the difference--SAAMI 12ga MAP vs 20ga--is only 500 psi (11,500 vs 12,000). That's nowhere near 40%.
Larry; First & Foremost that 11,500 vs 12,000 is Max Peak Pressure "NOT" Max Average Pressure. Average pressure will be way lower than the Peak. Using my same example if one pushes an oz of shot out the end of a barrel @ 1200 fps it takes a certain amount of work to be applied regardless of the size of the bore. In a shotgun the pressure rises very rapidly to its peak while still in the chamber & then falls, also quite rapidly at first & then slower as it goes down the barrel, giving the typical pressure Curve I am certain you have seen many times. The Average pressure is the total pressure under that curve. That work which is done, or Force, is proportional to the pressure times area. Since the 12 gauge has 40% more area to push on the 20 gauge has to use 40% more pressure to accomplish the same Force.
Again. FORGET the Peak pressure here, we are talking about the entire pressure curve for the total length of the barrel. Even within the same gauge, one can push the same shot load to the same velocity with widely varying Peak pressures. Keep in mind though one has not actually Lowered the average pressure, only Re-Distributed it.
An older Hercules handbook I still have has a chart showing the length of the column for one ounce of shot in the various gauges. for a 20 gauge they show 0.968" & for the 12 0.690". Divide .968 by .690 & guess what you get, a 40% longer column. This based on an ounce of shot occupying 0.288 Cu In of space.
That same 0.288 CuIn will also, depending upon exact granulation & density, contain 2˝ drams of Black Powder.
