The inside scoop on +P
Most shooters know the "P" in the +P designation on a cartridge headstamp stands for pressure and indicates that the cartridge is loaded to higher chamber pressures and thus higher velocities. But many are confused as to exactly how much pressure is added and how safe these high-performance loads are. I believe this confusion is contributed to by people in the industry, some by firearms companies that understandably wish to err on the side of caution in our litigious society and some by small ammunition manufacturers looking for an edge in a highly competitive market.
Plus-P loads can, in some cases, boost the velocity of short-barreled .38 handguns enough to ensure reliable expansion. However, they do result in added stress on a firearm.
The +P designation came about for a very simple reason. As advancements were made in the quality and strength of both firearms and cartridge cases it was determined that some of the older rounds were capable of operating safely at higher chamber pressures in modern firearms than those originally established. Since firearms--and cases, for that matter--are durable goods that last for decades, even centuries, it was not feasible to simply increase the standard pressure specifications for these cartridges. There are too many old firearms around that could not handle the increase safely. So SAAMI (Sporting Arms and Ammunition Manufacturers' Institute) uses the +P designation to separate the new pressure limit for these old cartridges from the old lower standard.
It is important to understand that SAAMI is the principle organization in the United States actively engaged in the development and promulgation of product standards for firearms and ammunition. Ammo specifications are not overseen by the Consumer Product Safety Commission or any other branch of government. Consumers should be aware that only manufacturers that are members of SAAMI are bound by the Institute's guidelines.
All the major American ammo manufacturers are SAAMI members, and most smaller outfits also abide by SAAMI guidelines, but I have seen ammo from one or two small manufacturers offered in calibers such as .40 S&W and .357 Magnum with the +P designation. Since SAAMI does not specify +P ratings for these cartridges there are only two possible explanations. Either the ammo is loaded to higher pressures than SAAMI deems safe or the +P designation is just marketing hype. Be aware, all comments in this article regarding the safety of using +P ammunition are related to SAAMI-sanctioned +P loads only.
Perhaps the careless use of the +P designation contributes to the caution on the part of some firearms manufacturers. Some manufacturers make vague statements in their owner's manuals regarding +P ammo that I feel adds to consumers' confusion. For example, some manufacturers of 1911s state, without explanation, that +P .45 ACP ammo is not recommended for use in short-barreled models. This is not due to any concern over chamber pressures. All modern 1911s in proper working order will safely handle +P pressures. The concern is the increased slide velocity produced by the hotter ammo, which affects the functional reliability of the handgun. These short-barreled variants of the 1911 are sometimes finicky, and a recoil spring tensioned for a particular power level of ammo helps to ensure complete reliability. Loads beyond this power level are not only more prone to jam, the added recoil causes more stress on the frame.
Another area of concern for some shooters is with the .38 Special +P loads and small-framed double-action revolvers. These little snubnose .38s have long been popular due to their light weight and concealability but are necessarily not as strong as beefier models. This is especially true of older handguns that may not have the quality of steel available today. I do not know of any current models in production that are not OK'd by the manufacturer for use with +P ammunition, and frankly, if I did I wouldn't fire the thing with any ammo. As stated earlier, SAAMI-specified +P is simply a modern standard for maximum pressure in these veteran cartridges, so if a newly manufactured handgun will not handle these pressures, I want no part of it.
I would be remiss not to discuss another P rating: +P+. This designates that the cartridge is loaded above SAAMI specs for +P ammo, and most manufacturers restrict sale of these loads to law enforcement, for good reason. These loads are carefully tailored for modern service handguns and may not be safe in all firearms. Thus they are not offered to the general public.
I feel we in the industry should make an effort to demystify +P loads. They are not, as some shooters believe, loaded to borderline pressures. The increase in pressure is moderate. For example, +P .45 ACP ammo is loaded to a maximum pressure of 23,000 psi compared to 21,000 psi for standard loads. Compared to the maximum pressure of other autoloader rounds, these pressures are very mild. The maximum pressure for the .40 S&W, for example, is 35,000 psi.
Any increase in pressure and velocity, however, does put more stress on the firearm. For this reason I use +P .38 Special ammo sparingly in my Chiefs Special, and my 1911s have heavier-than-standard recoil springs. It just makes sense to minimize stress on the firearm as much as possible. I am fond of my handguns and want them to last.
Plus-P ammunition can raise the performance bar for your handgun, but a tradeoff is more recoil and muzzle blast. In some cases it is worth it, such as when a little more velocity is needed to ensure reliable bullet expansion. Velocity increase is modest, however. On average, +P ammo is about 50 to 100 fps faster than standard ammo, sometimes less. In fact, I have encountered some +P loads that were slower than some standard loads available. As always, choose ammo wisely based on your needs.
The shooter considering using +P ammunition should follow the same safety precautions advisable with any ammunition. Make sure the firearm is in excellent condition and is approved by the manufacturer for the ammunition. If it's an autoloader, make sure the recoil spring is properly tensioned for the ammunition. And make sure the ammunition is from a reliable source and loaded to SAAMI specifications.