Join Date: March 2, 2005
Location: Central Texas
luvtoshoot, I'm sure you have a better handle on it now, but a similar question was posed in the pistol section. Since all I have to do now is copy and paste, I'll post it here. It is very long and was in regard to the 9mm, but the issues are the same:
I'll apologize in advance if I make this too long! I don't remember the exact year so lets say +/- 1990, SAAMI reduced the original pressure rating of the 9 x 19 Parabellum. They also changed the system used for measurement from Copper Units of Pressure (CUP) for maximum average peak, to PSI in which a Piezoelectric transducer measures chamber pressure. The Europeans measure in PSI also, but at the casemouth and is governed by CIP. I mention this because the CIP system is actually pretty close to the CUP system in value.
Okay, the original pressure spec of the 9mm was 35,700 CUP and there is no direct correlation to PSI, but I'll tell you that it is around 38,500 PSI (Pzo) according to Sturm, in the SAAMI system. Now you're gonna love this part! SAAMI decided to lower pressure to 35,000 PSI which is 33,000 CUP according to the ballisticians that use both measurements since they don't correlate. Lowering pressure also reduced the velocity of manufacturers ammunition. Enigma! People started complaining about the lower velocity they were getting in their ammo after this happened. SAAMI (which is mostly made up from the big ammo and gun manufacturers, i.e. Winchester, Remington and Federal) decided to offer a load with performance that was similar to what they had previously reduced and they called it +P. It had already been established with the .38 Special. The SAAMI maximum limit for +P is you guessed it, 38,500 PSI and I'll explain in a minute how I figured this out.
There are a few very skeptical folks in the gunwriting business and some have concluded that the P in +P should stand for Pink as in Pink Elephant! Because they feel that it more accurately describes a level of VELOCITY rather than PRESSURE. Then comes along +P+, first and mostly only available to law enforcement agencies. Then the same thing happened to .45 ACP and people began asking what the pressure level of +P was and were told that in 9mm is was 38,500 PSI so someone asked, Okay, then what's the pressure of +P+. Guess what? SAAMI has no established pressure maximum for +P+ so there answer included that and the rather bright observation that it is higher than +P, duh! Now many people are skeptical.
The gunwriter, Ed Sanow has touched on this many times especially in regard to his work on one shot stop data (take it or leave it, makes no difference to me) that some are critical of. I don't take it for gospel but it does have merit and some of the other scientific studies are even more flawed. There is a system that he and Marshall use in predicting the ability of a bullet to stop (I think it's Relative Incapacitation Index) an attacker by measuring temporary stretch cavity and penetration depth along with expansion in ballistic gelatin. I believe this is the most accurate data that is available to us and the predictions actually run a couple of percentage points BELOW the actual percentages in one shot stop data. Remember, you still have to accurately place the bullet! I mention all of this because it is the basis of Ed Sanow's skepticism and it would seem rigthtly so. The best performers in 9mm shootings and in ballistic gelatin are the Federal and Remington +P+ 115 gr. JHPs @ 1250 FPS from a 4" barrel. Sanow knew full well that a +P level load should easily be capable of 1250 FPS and in older load manuals you will find them over 1300 FPS. Since the introduction of new powders here and from Europe there are a few that will get to 1400 and a couple that will exceed it. DON'T TRY IT, you have to know all the specifics to do it safely and you want find it, not even in the load manuals. Vihta Vuori published some very impressive data when it first became available in the U.S., but they conformed to SAAMI test standards and lowered data even though they still use the CIP system, they currently hold data to 32,000 PSI (CIP).
I was able to approximate pretty closely what these values are by using the minimums and maximums in both systems and check them at points where they are given in both systems like the case of the current spec for the 9mm at 35,000 PSI and 33,000 CUP. The highest pressure used in U.S. commercial cartridges is 65,000 PSI which is the same as the CUP measurement of 54,000. The original spec of .270 Winchester. I have a good number of years using graphic geometry and I arrived at my conclusions graphically. Maybe because I heard early on in my education that anything in geometry can be solved graphically and I bought into since before computers and CAD, most of my working hours were spent on the drawing board.
Probably way more information than anybody needed, so just remember standard pressure limit for the 9mm, 35,000 PSI. Pressure limit for 9mm +P, 38,500 PSI. Pressure limit for 9mm +P+, none established but above +P indicated, nor likely needed since improved powder technology allows these velocities to be achieved through powder selection. The 10mm, .40 S&W and .357 SIG all arrived after the adoption of the PSI (Pzo) system so no +P is applicable even though there are a few wingnuts out there selling .40 S&W +P, some of you may have seen such, it is completely bogus and I wouldn't buy any of their products on general principle. Class dismissed!
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