Understanding SAAMI Pressures, or not?

[Stillhunter]

So, looking at the SAAMI website at the .358 Winchester I saw they do not have a pressure listed for it and many other cartridges. "Piezo pressure not established". I assume they are, retesting with modern equipment and have not got to some yet. I, in my very limited reloading knowledge assumed a .308 based cartridge would be the same as another, in modern designed rifles. But, 7-08 and .308 are 1000 psi apart.
Why in a modern firearm, would one cartridge be able to be higher pressure than another, some drastically?
I also wonder why say a 45-70 is loaded to 3 different levels based solely on the gun, i.e. Trapdoor, Marlin, Ruger Number 1. So, it is not the case design? Just the action strength no?
In a nutshell, why could you not load any round to the highest pressure the action is chambered for?
I ask out of curiosity. I have only loaded for a few calibers and always following loading manual spec's.

 

[Guy Miner]

Staying within the specs of published loads is a good idea.

Yes, the .45/70 is kind of a unique situation. Introduced all the way back in 1873, the big ol' blackpowder case was used in firearms that were NEVER intended for the pressure which would be just fine in a Marlin 1895 or a Ruger Number One. That cartridge's limits are absolutely based on the strength of the rifles used.

Why the 1,000 PSI difference between the 7-08 and the .308? I have no idea. :grin:

Some cartridges seem (to me) to be held artificially low. Such as the .280 Rem. I understand that's because so many of the slide-action and semi-auto Remingtons were chambered in .280, rather than the stronger bolt actions used for most similar sized cartridges like the .270 Win.

It's up to us as handloaders to determine what's an acceptable load for us! Tread with care, and keep using the guidelines of credible published sources. I find Nosler, Sierra and Hodgdon are the sources I use most often.

Regards, Guy

 

[DrMike]

The differences in pressure are not as great as they sound. When a cartridge is standardized, the ballisticians will recommend a maximum pressure. We are individuals, so the limits will likely vary. The difference in velocities with 1000 psi is pretty minimal--often in the order of less than 10 fps. Guy's recommendation to stay within SAAMI pressure limits is sound advice. Whereas some rifle cranks may tickle the dragon's tail, if they fail to watch the signs of pressure, the beast will turn and bite the rifleman. As a rather broad rule of thumb, brass will not come apart at under ~80K psi. Most modern rifles will maintain integrity up to ~160,000 psi. Clearly, there is a measure of safety built into those figures. What isn't apparent is that brass fatigues, as does steel. After repeated firings at over pressure, the shooter runs the risk of a barrel surrendering to the inevitable with catastrophic consequences. I really hate having a bomb go off four inches in front of my eyes. Even if the action and barrel maintain integrity, a ruptured case is no fun when hot gases vent out in front of your face. Just my ten cents (what with inflation and Obamacare and all).

 

[Stillhunter]

The 280 is a good example. Clearly it can be loaded to the same speed and pressure as a .270 in a modern bolt action. I see Bullet is loading the .358 to good speed and have seen John Barness loads with Tac so I can surmise one might approach this carefully even though Ramshot does not publish tested data.
Still, I wonder why with the caveat of MODERN BOLT ACTION, why all loads could not equal the highest pressure cartridge available in that rifle. Like the 45-70 loads come with that warning. I have no need to push anything, as I could not be certain of noticing an impending overpressure until I am already in dangerous territory. Sticky extraction, flat primer etc. All this is more curiosity and a learning experience.
Reading on here, no one seems glib or reckless in what they post, as on some other sites. Am I wrong believing that in a given caliber, and bullet weight, all other things being equal, same pressure = same velocity?
Perhaps I risk belaboring my ignorance. If so, I apologize.

 

[joelkdouglas]

I'm pretty sure I have loaded my 30-06 above the SAAMI max 60,000 PSI. I've only seen ejector marks once (Oops! I pulled the rest of those!), but I've gotten some velocities over published velocity data, so that means I'm over the SAAMI max.

But if a 270 Win can be above 60,000 PSI, no reason the old war horse can't do it with a similar action...

 

[DrMike]

You are not showing ignorance nor showing pugnacity; neither are you being particularly obstreperous. I take it you are questioning the reason for why things are as they are; and most of us have asked that same question at one time or another.

The .280 was loaded to a lower pressure because of the rifle in which it was first offered. It is generally conceded that modern rifles and brass will tolerate higher pressures. For instance, a rifle chambered in .280 Rem, sporting a 24 inch barrel, loaded with a 140 grain Nosler BT and charged with RL22 to near SAAMI pressure limits (loaded to 57806 psi) will delivered 3080 fps. Loading so as to approach a pressure of 64000 psi, velocity increases by 84 fps. For two grains more powder and 6000 psi yields a much smaller velocity gain than we might imagine. In the real world, there may not be enough velocity gain to justify the extra pressure. However, if the shooter is reaching for maximum velocities, he/she may conclude that the extra charge is justified.

Lever guns generally will not tolerate the higher pressure encountered in bolt actions. When a cartridge is standardized, the possibility that the cartridge will be fired from a weaker action is taken into consideration. Many older actions should not be stressed at all; the steel used cannot tolerate the pressures tolerated by modern steels. Again, this is factored in when a cartridge is standardized.

The final point is that though there is a correlation between velocity and pressure, it is not absolute. There are many other factors to take into consideration when building a load.

Hope that helps.

 

[Guy Miner]

Read one heck of an article about all this pressure/velocity stuff from John Barsness a while back. I have no link to it sorry.

Came away with one thought that seemed very important to me. I don't have his exact words but it was something like:

If you're getting velocities substantially above published max velocities, you're generating pressure well above maximum too.

I've wondered about that a time or two when I've seen some of the loads/velocities posted here and elsewhere. Many of us can accurately measure velocity, but can only guess at pressure.

Guy

 

[Stillhunter]

Thank you all. I was not understanding the diminishing returns of pressure to velocity. I was thinking both would be lineal. That seems to answer the questions I had.

 

[DrMike]

Thank you all. I was not understanding the diminishing returns of pressure to velocity. I was thinking both would be lineal. That seems to answer the questions I had.

Pressure is rising exponentially while velocity is plateauing.

 

[wildgene]

Thank you all. I was not understanding the diminishing returns of pressure to velocity. I was thinking both would be lineal. That seems to answer the questions I had.

Pressure is rising exponentially while velocity is plateauing.

...you end up w/ a lot more variables than case design like efficiency, dwell time, & pressure profiles, but @ some point you start hitting diminishing returns. A .300 WSM requires about 15% more powder than a .30-06 to gain 8% more MV. To gain 8% more MV over the .300 WSM requires 40% more powder in a .30-378 Wby...

 

[baltz526]

obstreperous.

To Roar at, To make a load noise, Noisy, Boisterous, Or Unruly, Esp. in resisting or opposing. SYN = Vociferous. You got me on that one, Had to look it up.

 

[350JR]

I'm agreeing with Mike, even if it doesn't sound like it to some but I may be approaching it at a different angle a bit.

SAAMI "maximums" ( there is more than one BTW) are the results of tests ran on AMMUNITION (not case strength) sent to them and that ammunition was loaded by the company issuing it for testing either for a PARTICULAR rifle or set of rifles they were coming out with at that time and/or could be used in many varying types of firearms even of different vintage.

The MAP they get from those tests create STATISTICAL (arrived at mathematically) other values (also "maximums") which even in name are "Probable" (vs possible). Even in these higher "probable maximums" each and every one contains the word "average" or "mean" indicating there exists or is expected both higher and lower pressure examples........ALL of which "pass" if only so many of them exist in a "lot" or "sample" tested. SOME of these OTHER "maximums" are several K psi higher than the "maximum average pressure" that, from what I read, most ASSUME is some kind of brick wall that beyond such all H will break loose.

These are just some of several things needed to be considered other than what RIFLE the loads were loaded for initially or the ones that MIGHT be around yet to shoot the ammo.

Simply put, a single "maximum" of those listed by SAAMI is considered by most to be THE maximum, which it is not by any stretch of the imagination. By SAAMI's own description it is basically the "target" pressure for manufacturers who submitted the ammo for testing to shoot for to duplicate such on a larger scale. In effect , like the "lawyer triggers" we have, the MAP is the "lawyer pressure" SAAMI suggests which is GOTTEN FROM the ammunition they were sent.....NOT in any way the maximum pressure the CASE will withstand as so many erroneously assume.

There are MANY examples out there making this absolutely clear.
As mentioned the 30-06 family of rounds do not share an identical SAAMI stated "pressure limits" in their MAP. The same situation is obvious in the 308 family, and the 358 mentioned is one of the worst "under rated" of that family if looking at SAAMI statistics.

Even with these staring us all in the face for decades and the fact that there are examples of many older rounds that are the "parent round" of many newer rounds of MUCH more chamber pressure it is common for responses to any load one may share that scream the poster is "LOADING WAY OVER SAAMI MAXIMUM" and feels the poster is literally taking their life in their hands when they just SAID they are not experiencing ANY type of "pressure sign" from flattened primer to head expansion to bolt lift being sticky to an unexpected spike in velocity.

In actuality NEITHER the loader or the person responding have anything more than an educated guess at what the ACTUAL pressure is, let alone what maximum is for that set up, unless someone has the equipment and expertise (within the link below SAAMI makes a point of listing several things that can and do effect their testing) to test that powder charge, in that powder lot, in that rifle, with that bullet, in that bullet lot, within the present atmospheric conditions, with that particular cartridge case or group of such in that lot of cases.

Case capacity, chamber dimensions, bullet seating depth and bullet style, bore condition, primers, propellant lot characteristics, atmospheric conditions and a number of other factors all affect the ACTUAL pressure one is shooting from any load. One can take the exact information from a handloading chart and use the exact same components with the same brand and model of bullet seated to exactly the same depth and there is exactly ZERO chance that the actual pressure it produces will be that which is listed in the chart. There simply exists too many variables, all of which effect actual pressure and this is the REASON we all start loads a certain percentage below the STATED maximum. It is possible that for many rifles maximum will be reached BELOW the stated maximum in the chart. It's not rocket science to conclude that there is ALSO rifles and component etc etc combinations where the stated maximum isn't the actual one yet the masses want to believe that the stated maximum charge applies to all rifles all the time.

IMHO the best examples that SAAMI and even CIP "maximums" are not CASE limitations across the globe is the 6.5x55 Swede 7x57 Mauser and 8x57. Each, having been around since the end of the 1800s, has rifles existing from that era that MAY still be in use and those vary too in condition therefore factory ammo is loaded to sell to the public that the manufacturer feels they are safe from being sued for blowing up the poorest of condition of the weakest version of the rifles out there chambered for such. Those loads were sent to the testing organization for that country and what those purposely low pressure loads came out to be.........became the "maximum pressure" for that round in the eyes of the general public.

To make it even MORE confusing for those going by ONLY the published maximum chamber pressure, SAAMI fearing law suits more than CIP obviously, list most values lower than the CIP value (for the same round). CIP governs (not suggests but actually CONTROLS such) the 14 countries under it's realm. In the case of the 8x57 it's a LOT lower in SAAMI than CIP.

SAAMI states the
6.5x55 SE and the 7x57 are 51,000 MAP in psi
for the 8x57 it states 35,000 psi for the MAP.

CIP listes the
6.5x55 SE at 3800 Bar (55,114 PSI) with a "maximum statistical individual pressure" of 4370 Bar (63,381 PSI)
Both the 7x57 and 8x57JS is given 3900 Bar and 4485 Bar for the MAP and "maximum statistical individual pressure". That's 56,564 PSI and 65,049 PSI.

Those pressures are where the companies governed by CIP load their ammo. This ammo is in a lot of the same cases we shoot here AND in ammo that often can be bought here and they have the same "old rifles" in use there that we do. Quite obviously someone telling me some arbitrary "chamber maximum" based on whichever SAAMI limit they wish to pick from the chart is quite obviously incorrect

Some may want to jump in here with "yeah but CIP tests differently" which may be true BUT if you compare SAAMI specs to CIP specs for a large number of rounds you will find more than a couple that have the same rating or something very close. They are not radically different and certainly not the amount shown by these examples.

Keeping in mind that the countries that CIP controls is where these rounds came from initially and the fact that they certainly HAVE TO have a ton of old military guns around chambered for these rounds that are also common here in the States, one has to realize that the statistics shown on SAAMI spec charts are not any kind of "maximum" at all, generally speaking.........for any round.

As mentioned above, unless you gentleman have some way of knowing the ACTUAL pressure a load produces in a particular rifle that you are not sharing......we simply do not "know" what it is NUMERICALLY. Load data is our method of knowing where to "start low" and work up.....slowly and pay attention. Once ANY kind of pressure sign appears each of us decide how much we wish to back off for general hunting loads. That amount varies too depending on how insistent a person is to squeeze every last fps out of the round.

Personally, I'm a little like SAAMI. If I'm going to err it is going to on the side of caution. REGARDLESS of what actual pressure IS, what load combination fires repeatedly without pressure signs offers me little in reality compared to what another load a few tenths of a grain lower. Trajectory wise to the animal being DRT that little extra just wont be realized in the field, therefor unjustifiable to me personally.

The load data charts and the QL results give one a good perspective of what powders supposedly give the best velocities with the least pressure and gives a person a good idea where to start loads to work up and IMHO........nothing more.

What the rifle and load indicate is such while working up loads is the REAL "maximum" and all of us know it varies dramatically sometimes gun to gun. This is just yet ANOTHER clear sign that the numeric value put on "maximum chamber pressure" printed on any form or any website is nothing more than a statistical guess. Those who quote "I'm shooting 64,000 PSI loads" are assuming things that are impossible to know making the statement totally moot.......and quite probably incorrect.

The big issue is to test and retest whatever load we use and whenever a single component changes.......start over. Making sure what we feed our rifles is safe in OUR rifles is the whole ball game. What may or may not have been tested that is similar in another rifles is pointless to consider as any kind of maximum OR even safe to shoot in ours.

For those not having read the entire publication from SAAMI it is informative, but quite long as well and I include it here only as shared information. As in all things your interpretation of it may differ from another's but it remains information few consider when grabbing a listed statistic from the SAAMI list offering it as a universal maximum for all rifles in that caliber.

http://saami.org/specifications_and_inf ... ad/206.pdf