Home Forums Hunting & Shooting Ask The Gunwriters Pressure increase, can you predict it?

Keep in mind as well that "average" can cover a broad range of individual shot pressures.

As an example, let's postulate that the SAAMI number for an imaginary cartridge is 60,000 psi (however we measure it).

We fire a ten-shot string with one low shot of 40,000 and one high shot of 80,000. The AVERAGE may still be 60,000, but continuing to operate with that load is going to have that occasional 80,000 psi shot. Not what I'd call a safe load, even though we think it's at the SAAMI average.

BTW, I suspect that this is where we get some of the differences in printed manuals. The test lab may adhere to the SAAMI average maximum, but may also add in constraints of their own, such as no one shot more than x% above average, or a spread of pressures no larger than Y, and so forth.

IIRC both Kens have touched on it in the past, there's no free lunch (fast barrels/slow barrels). If your velocities are over published maximums you may well be in the danger zone.

I've also worked with cartridges that would show little or even a decrease in velocity as the published max load was reached. There weren't any of the traditional pressure signs.

I worked up loads for a friends 7mm mag. He wanted me to load Barnes bullets for an upcomming elk hunt. The velocity of the recommended starting load was 200fps over the max load listed in the manual. You could push primers into the pockets after firing the brass once. I sent him on the trip with Accubonds (still had to reduce load). I trust my chronograph (course it's an Oehler)

Very true! And all the more reason for using an optimum powder at 90�100% case fill!

Extreme shot-to-shot variations epitomize inconsistency. The positions of a low-fill charge of powder in the case ([a] near the web, [b] along the bottom of the case, or [c] near the bullet), varying from shot to shot, guarantee extreme shot-to-shot variations. A full or nearly full case is a better assurance of moderate pressure variations.

Also, a slight variation in charge weight is a smaller percentage of a heavier charge. For example, half a tenth of a grain (0.05 grain) variation is a larger percentage of a 35.0-grain charge than it is of a 50.0-grain charge.

Rocky,

Zig Ziglar used to say: "If you put one foot in a pot of ice water, and the other in pot of boiling water, on average, you wont be comfortable."

Which brings me to the next logical question I have about establishing "max safe avg pressure". Obvously SAAMI didn't establish, with our "fictional 60,000psi round" a "max safe avg pressure" by getting 40K and 80Kpsi data and calling 60K safe. So, what are those standards? How small a deviation does it have to be? If you know.

This sort of swings us back around to my original question too...because those standards have to be set so that the "high end" is high enough to be a productive round, but not so high that it's right at the end of "the straight part of the road" where it starts to get real curvy. There has to be, as KenO suggested a little conservative approach such that the max safe avg pressure is just that from all perspectives "max" "safe" and a true repeatable "avg" with acceptably small deviations such that its not just a midpoint between to unreapeatable numbers.

Ken,

I'd never known you were a "full case" proponent...I've been stumping that for as long as I can recall. Still, so many I see, even here, are using powders and loads at 60% (or even less) density with very fast for cartridge/bullet wt powders. I even saw in the reloading forum someone talking about experimenting with and actually using loads of 22grs of Blue Dot with 100gr bullets in the .260Rem. It appeared to work for them...but I just don't see how you even start down that road.

I say "90�100%" as theoretical permissible range � but in practice, I seldom load below 97% or 98% case fill.

If I remember correctly, say a 60,000 would be the Max. Average presure. The Max pressure allowable in a test string may be 67,000 psi abosulute max., but the average would be 60,000PSi..

I like to hear a little crunch when I seat the bullet!

Good info to know. Is that from SAAMI?

Me too.. That sound means a proper ratio to case capacity has been achieved....

As I recall, the SAAMI limits reflect an assumed coefficient of variation of 4%, or that the standard deviation is 4% of the average. For the mythical 60K psi Maximum Average Pressure, this would mean that roughly one round out of a hundred might exceed 67K psi. I know of no published limits on the maximum allowed standard deviation. You can be assured that if the observed SD is too high, the test will be refired several times.

KenO

Great information, thanks!

280, I don't know what the variability standards are for SAAMI. That's why I suggested (and also strongly suspect) that the various labs establish their own standards within the SAAMI ones. So even though all the labs publish data that stays within SAAMI maximum average pressure, they may ALSO say that no load in the string may exceed the average by more than an percentage or by a set pressure number. If a test load in th string violates their added standard, they'll back down a bit.

And that may help explain why we may see a maximum load listed that only achieves (for example) 55,000 psi when the SAAMI max ave is 60,000. While another lab, with a different set of internal added test standards, lists a slightly hotter load with an average of 58,000.

Reloaders then ask which is right, or why the disparity. The answer, of course, is that they are BOTH right, but the maximum loads they publish reflect slightly different internal rules.

Also no 2 different batches of the same powder and shot in different rifles will achieve the same average pressure, and still stay below tha Max average pressure.... For differnt components and rifles to stay with-in 5,000 PSI is rather good IMHO..

Rocky, The A-Square manual states that CIP sets a maximum average pressure for a cartridge and uses a multiplication factor of 1.15 to set a maximum pressure for any one round in the sample. SAAMI sets a "maximum average pressure" (MAP), which is the the recommened maximum pressure level for loading commercial sporting ammunition. They also specify a maximum probable lot mean (MPLM), which "represents the midpoint of the upper service pressure distributaion." What exactly this figure is they don`t say. They do however show both figures in their data for each cartridge. The 270 Win for example shows a SAAMI max avg pressure of 65,000 piezo with MPLM of 66,600 psi or in CUP 52K CUP max - 53,300 CUP MPLM.

More interesting IMO, are the spreads between max avg and max allowed pressure some of the Weatherby cartridges show. These cartridges appear to be CIP governed. Take the 257 Wthby with a max avg of 63,861 psi piezo, the max individual allowed is 73,440 psi piezo!!
The difference shown between the SAAMI and CIP methods may account for the variation in different lab data if they use one standard in place of another, max allowed vs max pressure avg mean. This of course is nothing but speculation on my part.

Also, called SAAMI today. The gun manufacturers, to comply with SAAMI, a gun must proof out at 130-140% of SAAMI max for a given round. So a 270Win must proof out to withstand 84,500-91,000psi. Not for a string of shots, but it must withstand one and maintain it's intergrity.

That was the most interesting thing in the manual.I guessed that MPLM was tighter than what CIP does.

Ken's recollection is correct, as you would expect. SAAMI assumes that the standard deviation is 4%. I don't know many handloaders that are not more consistent than that.

Dr. Brownell did a graph of pressure vs. charge for a 30-06, covering very light charges through very excessive charges. The pressure graph is gently exponential. The rate at which powder burns is a function of pressure. As pressure goes up, the powder burns faster. So it's kind of positive feedback system, which results in the exponential increase in pressure vs. linear charge.

To some extent, the natural exponential nature of the pressure vs. charge curve cancels out the natural square root nature of the charge vs. muzzle velocity curve. The net result is that the MV vs. charge curve is more nearly linear than you would otherwise expect. Still, the linear approximation is best used over fairly short stretches of the curve.

You can reach a point (still ruminating over the exact mechanics) where pressure continues to go up exponentially, while muzzle velocity flattens out. Had that happen with my 308 and 2520 powder once--below a book published load.

So... the linear estimate works... except when it doesn't. Which is often enough to make things a kind of interesting that you might not like.

SAAMI does publish three different pressure specs for each cartridge. There is the higher real spec, and there is the lower MAP, which we load to. MAP recognizes that a 10-shot sample will not give you a perfect estimate of mean pressure, and backs the allowable measured pressure down two standard errors to insulate against sampling error. This is extremely sound practice.

Good info and excellent read. Mechanics of a charge/bullet and chamber are a lot more complicated than most people can grasp. But why did Roy after reaching his max charge start free boring to increase MV?
Don't want to hijack but seems that it could fit in this discussion. Have a great day guys.

Free bore allows the bullet a slight run before engaging the rifleing, and allows the presure to peak slower, think of it like a slight case capacity gain...