By Tom Roster
There are two important ballistic measures for every shotshell load. The external one is velocity; the internal one is pressure.
Photograph by Terry Allen
Many shotgunners rely on the information printed on cartridge boxes or in manufacturers’ catalogs or websites to tell them the velocity level of a given load. In handloading, the information goes a big step further by also listing the chamber pressures of the various recipes. With factory loads, however, chamber pressures are almost never listed. Consequently, I have found there to be a great deal of ignorance surrounding shotshell pressures, especially “high” versus “low.”
High pressure takes on an element of perceived danger, because many shotgunners have been taught that so-called high pressures can cause shotgun barrels to “blow up,” bulge or split. This misunderstanding stems from the failure of those discussing or writing about the subject to differentiate between high versus excessive pressures. SAAMI in the US and CIP in the UK and European countries establish maximum pressure standards for hunting and target loads to be used in shotguns. Maximum pressure allowances increase as gauge decreases (e.g., from 12 to 20). SAAMI and CIP also establish standards for the pressures to be developed by proof loads for the purpose of testing shotgun strength. The standards from both governing bodies are similar but not the same. And I doubt they ever will agree to make them the same.
To get a handle on high versus excessive pressure, let’s look at 12-gauge 2¾" loads. The maximum average pressure (MAP) permitted by SAAMI for 12-gauge 2¾" service loads is currently 11,500 psi. CIP permits slightly less at 10,730 psi (740 bar). (A complete delineation of the standards—both MAP and definitive proof levels—from both entities for all gauges and shell lengths was presented in my Nov/Dec ’13 column.) High pressure is anything within about 2,000 psi less than the upper limit of the MAP standards. Only pressures exceeding MAP standards would be excessive pressures. The more that excessive pressure exceeds the MAP, the more potentially damaging it becomes.
To be clear: There is nothing dangerous about high pressure levels in shotshells, only excessive pressure levels. As long as the pressure developed by the load is within the MAP and the firearm it is to be shot in has been proof-tested to handle that MAP, there is no problem. Some shotgunners have deluded themselves into thinking they don’t want to be shooting what they call “high pressure loads.” Since pressure levels are not printed on shotshell boxes, factory-load shooters wouldn’t know anyway. But some reloaders with access to pressure levels published for all reloading recipes think they want to be shooting only recipes that are a couple thousand pounds or so below the MAP for each gauge and shell length. When asked why, they often answer with things like “it’s safer” or “there’s less recoil” or “they pattern better.” Thus they voice three fallacies.
First, in a 2¾" 12-gauge shotgun proofed to SAAMI standards for 11,500 psi MAP service loads, there is nothing safer about a 9,500-psi load than an 11,000-psi load. Second, pressure levels have nothing to do with the amount of free recoil generated. They can only (and this is highly relative to each shooter’s sensitivity to recoil) diminish the sensation of felt recoil when certain slower-burning powders are used to achieve the same velocity level with the same load weight. But most shooters in blind tests cannot accurately detect differences in felt recoil. Last, I never have been able to measure generally better patterning performance from lower-pressure loads. But I definitely have measured generally better patterns from lower-velocity loads.
How about so-called low-pressure loads? To many shotgunners this is some undefined but desirable pressure level assumed to automatically produce the dubious benefits detailed but rebutted above. There is also the unscientific and never-tested belief that pressure levels less than 7,000 psi automatically make loads OK to fire in old shotguns. Such loads (often dubbed “vintage”) also commonly are assumed safe for use in shotguns that owners are concerned may not be compatible with the pressure levels developed by modern smokeless-powder shells—especially American loads. Be advised that until SAAMI and/or CIP develop pressure standards for vintage-type loads proven and tested safe for use in such shotguns, the 7,000-psi pressure level and this practice will remain purely speculative.