Interesting read by Randy Wakeman
SAAMI Shotgun Pressure Specifications
Shotshell Maximum Average Pressure (MAP) in PSI
10 gauge 11,000
12 gauge 11,500 (except 3-1/2 in.)
12 gauge 3 1/2 in.14,000
16 gauge 11,500
20 Gauge 12,000
28 gauge 12,500
.410 Bore 2 1/2 in. 12,500
.410 Bore 3 in. 13,500
Pressure is discussed a great deal in a casual sense, but like most things unseen and unmeasured by most . . . little valuable information is normally imparted.
Lab data does not measure or indicate pressure in your gun. In the case of many factory shotshells, specific pressure is not discussed. In some cases, it cannot be reliably mentioned, as shotshells are marketed on the basis of performance, not SAAMI pressures. Shotshell manufacturing is a very competitive business; some shotshell makers (like rifle cartridge makers) have multiple approved recipies for the same shell-- the "why" is so they can buy and use whatever powder, many of them bulk or proprietary powders, that is found to be the most economical at the time. It could be St. Marks Powder one day, Nexplo the next, and ADI for the next run.
With shotshell chambers varying in length and diameter, exact pressure cannot be predicted. A simple primer change can change pressures by 3000 PSI. My friends at Accurate Powders / Western Powders inform me they have seen 5000 PSI changes just by a primer swap through their test barrels as recorded by radial tranducer.
Peak Pressure has no relationship to shotgun recoil, and so is not part of any free recoil formula. As published by Lyman and other sources: E = 1/2 (Wr / 32) (Wb x MV + 4700 x Wp / 7000 x Wr)squared.
Where E = recoil Energy in ft. lbs., Wr = Weight of rifle in pounds, Wb = Weight of bullet in grains, MV = Muzzle Velocity of bullet in feet-per-second, Wp = Weight of powder in grains.
Pressure, peak or otherwise, does not exist as part of free recoil. It likely won't stop those who think a tiny peak pressure node level equates to recoil, or enjoy speculating that that there is a relationship-- but there is no basis for it, unless we have a brand new branch of physics that attempts to now address it..
Peak Pressure, that always happens inside the shotshell, also has no relationship to pattern quality. Peak Pressure must always follow the base of the wad. When "pressure" is discussed, it invariably means only the peak pressure node that exists for mere fractions of a thousandth of one second. As its existence is so very fleeting, it cannot and does not illustrate the entire pressure curve-- nor even a substantial portion of it. Oberfell, E. D. Lowry, Zutz, Brister, Brindle ... no one familiar with exterior shotshell ballistics has postulated, much less shown, that peak pressure does anything to patterns one way or the other. If you follow the logical continuation of the discussion, a reduction of 3000 PSI must equate to something in pattern percentage. It doesn't, not one percent, and so the entire discussion moots itself in one big hurry. There is also nothing to show that higher peak pressure means more "open" patterns: if it did, every skeet load made would be at the highest MAP pressure possible-- quite obviously. Naturally, they are not. The peak pressure always happens inside the shell, before shot touches the first forcing cone, or the second forcing cone we like to call the "choke."
The bump up in pressure in the "newest" popular chambering, the SAAMI 3-1/2 inch 12 gauge, allows the 3-1/2 in. 12 gauge to effectively obsolete the 10 gauge when used with steel or other no-tox shot not subject to deformation. It allows a payload (or velocity) increase for the gauge beyond what can be had with the old 12 ga. 11,500 PSI MAP limit.
We all like to think of "pass / go" and "good / bad." We also have difficulty accepting that a "pressure number" is a vague one, contingent on our gun, our ambient conditions, and tolerance stack-up. Wad material variances affect pressure, crimp depth affects pressure, hull basewad variances affect pressure, lot-to-lot powder variances affect pressure, lot-to-lot primer variances affect pressure as well. It is a very vague if interesting number, unknown by most shooters in their guns with any precision, and has no effect on recoil, patterns, and assorted other attributes attempted to be associated to the tiny, fleeting little peak pressure node.
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