Why So Much Concern About Pressure In 40 Cal?

[thefish]

It seems to me that the topic of high pressures and the problems it causes comes up more in this cartridge than others when compared to 9mm or 45ACP. So, I looked up the general range of CUP in these 3 cartridges in Lyman's 48th Edition and this is what I found:

9mm: 27-32K CUP

40 Cal: 20-23K CUP

45 ACP: 10-15K CUP

These are my questions:

1) Is my basic premise wrong and there really is no inordinate concern about pressure in 40 Cal over these other pistol cartridges, and high pressure, when unchecked can lead to potentially VERY serious problems in any gun?

2) If not, why isn't there more concern over the 9mm, which has the highest pressure of all three?

3) If so, what is it about the 40, i.e., is there some special feature of the 40 that makes it have high pressures relative to its size?

4) Is the high pressure of the 40 cal (or 9mm) of more concern in certain guns than others?

This entire matter of pressure has confused me more than any other topic in the world of reloading. I'd appreciate any thoughts, opinions or ideas.

Thanks.

Edited September 8, 2006 by thefish

[EricW]

Your reloading manual is in err or you are interpreting the #'s incorrectly. My Hodgdon manual shows 40 as being in the 32Kpsi to 34Kpsi max pressure range....just like 9mm.

The danger of 40 is in the following:

- 40 is generally chambered in firearms designed for 9mm. There is less steel as a result when that firearm is converted to 40 by essentially boring out the chamber and barrel.

- 40 cases are larger in diameter than 9mm, thus there is more force on the remaining steel. (psi x area = force). The 40 is also longer...and thus more force.

- The unsupported case web is larger thus creating a longer "cantilever beam" that the brass must provide in order to not burst.

So, when you bore out a barrel to go from 9 to 40, like virtually EVERY firearm on the market was engineered to do, you have three major engineering factors working against you to create much more stress in the steel.

Just accounting for the difference in bore diameter. If the barrel hood is .700" wide, switching from 9 to 40 reduces the strength by 7%. Accounting for the increase in surface area, assuming equal case lengths, you have effectively reduced the strength of the barrel (i.e. increased the stress) by 14%. Account for the increase in case length, and you're looking at maybe a 20% relative decrease in strength in a 40 bbl versus a 9mm.

The usual engineering safety factor for things that don't fly is 2. That means a gun that's designed to safely operate at 32kpsi, disassembles itself at 64kpsi. My guess is for firearms, because of liability, that the safety factor is in the neighborhood of 2.5 to 3. Let's call it 3. When you convert a 9mm firearm to 40, you've gone from a safety factor of 3 down to 2.4. If you end up through a series of metallurgical misfortunes to get a gun with a safety factor of 2 as designed for 9mm. In 40, you're at 1.6. Now, you have a pistol that disassembles itself at 51.2kpsi. Accidentally creating a round that is 50kpsi in 40 is a depressingly easy thing to do. A couple extra tenths of powder and a set back bullet will get you there quite nicely. What seemed safe before is now on the ragged edge of disaster.

Like it or not. It's the simple truth. This is why 40 blowups are relatively commonplace as compared to 9mm. Everybody was all a twitter about 9 major. Off the cuff, 9 major seems a lot more tame than 40 major from an engineering statics standpoint.

That said, I shoot 40 major and lots of it. Do I worry? Yes, a little. But I also have availed myself of modern reloading technology to mitigate the risk of overcharged ammo.

I also think that the cavalier attitude toward 40 as "just another cartridge" is pretty misguided. It is not. It's a high performance cartridge and just like a supercharged, high-compression ratio engine, it needs to be given the respect it deserves. Goose the throttle a little hard and life can get expensive in a hurry.

[XRe]

In addition to Eric's excellent disertation... In an (effective) attempt to get a recoil profile that is more manageable and competitive, most folks are loading heavier bullets in .40 (180-200gr range) using powders faster in burn rate than the various manufacturers recommend. What this means is that while the speed of the bullet may not increase, the spike in pressure during combustion happens more quickly than factory ammo, and probably spikes higher, as well. So, there's another engineering factor that's reduced the "headroom" in the cartridge.

While running a 180gr bullet at 170 PF isn't all that crazy in terms of the ballistics that .40 offers (for instance, factory WWB 180gr stuff goes about 1000 fps out of my gun...), doing it with fast powder greatly reduces that safety margin....

Open gun loads are generally loaded with slower powders than we're loading iron sight guns with - sometimes much slower than normal factory loads (for instance, loading Super w/ N105 or 3N38). In general, while the ballistics on these loads are quite impressive, they are much *safer* to load, as the margin for error with a slow powder is higher, and generally the gun is built to take it, as well (ie, engineered properly for the cartridge). Some 9mm Major loads I've seen have some, uh, interesting pressure signs, but generally speaking, appropriate powders make it as safe or safer than loading .40 - and we're talking +P+++ type pressures here, a large amount above 32K..... .38 Super open loads with slower powders give me a much bigger warm and fuzzy than even loading .45, so...

[BSeevers]

Yea what they said and Its easier to blow a .40

[Allgoodhits]

Excellent replies.

If one continues to ignore "pressure signs" they are destined to have a firearm failure. The questions are only when and how severe!

Pressure increases are not linear. One could go from just under maximum pressure to a catastrophic failure in a 1/10 grain or so depending on caliber, bullet, powder, chamber, OAL and how much the weapon has already been "stressed from previous testing" of over SAAMI spec ammo.

Be safe, don't ignore the manuals.

MJ

Edited September 8, 2006 by Allgoodhits

[Genghis]

Wow! Superb explanations.

I haven't reloaded .40 or 9, but doesn't the .40 have less empty space in the casing than other calibers with some powder/bullet combinations? That would help explain why a small variation in powder charge can produce a huge change in pressure.

[thefish]

Your reloading manual is in err or you are interpreting the #'s incorrectly. My Hodgdon manual shows 40 as being in the 32Kpsi to 34Kpsi max pressure range....just like 9mm.

Yes, I see. Thanks for your time. I checked the manual again. What I quoted was CUP and what you indicated was PSI. I knew that CUP and PSI are not transposable, but I thought that they were equatable, i.e., if one saw a high CUP number, that the PSI number would also be high, in a roughly linearly related, proportional way.

Am I wrong about CUP vs. PSI?...CUP should thus be considered an antequated system and viewed with much suspicion?

Thanks again, great answer.

[Steve Moneypenny]

Biggest problem with the 40 is bad brass and oval chambers. There isn't much of a problem with 40's blowing up any more that mostly went away with the 90's and the bad brass.

[Guy Neill]

I don't have SAAMI books at hand, but my recollection is that there is no CUP standard for the 40. All newer cartridges have PSI standards only. The older cartridges have both, but the CUP will likely be phased out. There is no valid conversion between CUP and PSI, but it will normally correspond that a higher PSI/CUP number will relate to a higher CUP/PSI number.

Working from memory (so do NOT take these as gospel), the 45 Auto is some 18,000 CUP, but 21,000 PSI. Similarly, the 9mm Luger is 33,000 PSI and 35,000 CUP. The 10mm is something like 37,500 for both. Again, these are numbers from memory and may not be the actual values, but it shows there is no one to one correspondence between CUP and PSI values, and other cartridges will differ, depending on the agency setting the values (the originator of the cartridge to SAAMI).

The 40 is a 30+ thousand psi cartridge, like the 9mm.

SAAMI members receive the full set of standards, but an abridged set is available through ANSI, in four volumes, one each for Rimfire, Shotshells, Rifles and Pistols. Something like $40 each. Someday I'll get a set for reference. They are revised/updated every few years.

Guy

[EricW]

The CUP system and the current system of piezoelectric transducers each have their quirks. The transducer system is better for technical reasons other than pure accuracy. It gives more information. Particularly, one can get a curve of the pressure over time.

The two systems are not interchangeable and as far as I know, there is no reliable conversion formula between the two systems.

And yes, the 40 brass blowout issue has largely been solved with beefed up cases. There are still a lot of old cases floating around out there, however. I think the moral of 40 is that it is a cartridge with which you approach your load incrementally. Just like 38 Super in major.

[XRe]

And yes, the 40 brass blowout issue has largely been solved with beefed up cases. There are still a lot of old cases floating around out there, however.

Good to note, too, that while beefed up cases may help w/ blowout (where the case ruptures due to lack of barrel support at the web, and vents downwards into the gun...), it doesn't make much/any difference for the much more violent detonation that occurs when load variables change radically enough - resulting in the proverbial "KABOOM!"... (ie, fast powder, heavy bullet, small amount of setback, and... blammo...).

Gotta be careful regardless of which cases you're using when playing with these recipes - and work up slowly, etc...

[Harmon]

I would add too that the 40 brass is probably as thin as 45 acp brass or old super brass while running 35K PSI .

you also run into problems with guys who reload glock and refuse to install a match grade barrel..more brass bulging!

[JFlowers]

If 40 pressure is a concern in guns that are basically 9mm frames bored out, is it that much less a concern in guns that are 45 frames? Like the 1911 family? It would seem that they would have much heavier chamber walls... unless I am totally missing something!

Eric, I mean 40 caliber guns on traditionally 45 frames, like the 1911.

[Patrick Sweeney]

A "scaled down" gun like a 1911, compared to a "scaled up" gun like a Glock/SW/ et al, only saves you on the margins. You can't do much about case blowouts. If the variables are against you you'll blow a case regardless of which pistol it's in. What the 1911 does for you is offer a bit more margin in the case blowout, perhaps avoiding trashing the gun.

But, usually when these things happen it isn't a partial event.