Home Forums Hunting & Shooting Ask The Gunwriters Loading manual pressure info?

My Lyman manual gives pressure info on some loads but not all. My Hornady and Nosler manuals don't give individual load pressure info. Is there a manual or other reliable source that gives estimated pressure info for standard loads?

Why would they give "estimated" pressure? And even if they did, how would it differ from the estimated pressures provided by the many handloaders who can apparently guess within 1000 PSI from looking at a fired primer?

When loading manuals list pressure, they normally measure it.

Any pressure data you find is gonna be an estimate, cause it ain't your gun.

How, exactly do you plan to use this estimated pressure data? Published loads should be safe in firearms in good condition provided you work up from the low end, don't substitute components willy-nilly, and don't exceed maximums just because you can open your bolt without a hammer. Within those limitations, the pressure is the pressure and knowing it exactly or estimated is of no particular advantage that I can see, except possibly in regard to case life. What matters is getting sufficient velocity and accuracy for the job at hand and going home with all your digits and eyeballs where they belong and not in a baggie.

Ok, estimated was the wrong word to use. Do you know of any manuals that give pressure measurements for all their loads?

If you use Hodgdon's online manual, you will get pressure data in either CUP or PSI.

http://www.hodgdonreloading.com/

Thanks for the link. Good info.

Lyman is the only one I know of, but I only use the Lyman, Sierra, Hornady, sierra and Nosler manuals.

I would like to see something in manuals stating what average maximum pressure they used to develop their loads. I think Speer does that for some rounds. One could assume the manual authors went by SAAMI standards but in the case of older rounds it would be useful to take them to pressures modern firearms are capable of.

E.g., "Ruger only " .45 Colt loads. Okay, that's nice, how about a blurb stating "these loads developed no more than 30,000 PSI in our pressure testing using the firearm noted in the cartridge notes". Or, these 6.5x55 or 7x57 loads developed no more than 60,000 PSI in our modern Tikka test firearm. If you are using a Model 96 or older pre-WWII firearm stop at these loads which developed no more than 51,000 PSI.

Extra work to be sure but it would certainly help those who want to take full advantage of older rounds in modern firearms without extrapolating blindly. I know you can sort of get there by comparing case capacities of similar rounds - 7mm-08 and 7x57 for instance, and loading to similar velocities, but hard data would certainly play to many a loony's (or loonie's) quantitative heart.



Added: the 6.5x55 is a good example. I have a 1915 M96 Swede and don't even approach max loads listed in the Hornady manual for the 6.5x55 which are much slower than those listed for the .260. But in a modern Tikka I don't see why one couldn't load to similar velocities as the .260 loads are showing.

Thanks, I use their hardbound manual and didn't know that either.

Good post.

43Shooter,

OK, here is some pressure info that might help:

Members of the Sporting Arms and Ammnunition Manufacturers Institute (SAAMI) voluntarily adhere to certain standards in guns and ammo, mostly so all ammo will be safe in all rifles made by members. All the companies mentioned so far are members of SAAMI.

The SAAMI pressure standards include Maximum Average Pressure (MAP). You can find the MAP for most cartridges one the SAAMI website, so you can assume that even though a company doesn't list pressures in it's load data, the MAP won't exceed that pressure. In fact, it's usually less than the MAP, because SAAMI standards also include a stipulation that any single round in the tested string does not exceed the maximum.

Loads vary somewhat in pressure over a string, some more than others, which is usually why the top pressure varies when a company does list it. This is also why a handloader can't assume that just because a load averages 62,000 PSI when the SAAMI MAP is 65,000 that there's 3000 PSI to play with.

Companies using piezo-electronic pressure equipment (the most accurate system presently available) do try to use test barrels with minimum-dimension chambers and bores. This means the typical factory barrel will usually produce less velocity and pressure, because both the chamber and bore are normally larger.

Most companies, however, use strain-gauge equipment to test for pressures, usually using a factory rifle. Strain-gauge equipment isn't as accurate as piezo, but is close if the same barrel is tested with SAAMI "reference ammo." This is factory ammo that's been tested in piezo equipment and produces X amount of pressure. The strain gauge will show a different pressure, but the results are adjusted ("offset") to come up with a close idea of pressure. (Strain gauges indicators do NOT show the actual pressure. In fact they usually show a somewhat lower pressure than piezo equipment, something many people--including a few supposed professionals, don't understand. Without reference ammo strain-gauge results aren't valid, except on a comparative basis: One kind of ammo produces more less pressure than another.)

Testing by SAAMI members is supposed to be done under very controlled temperature and humisity conditions, unlike the average handloader who tests outdoors under whatever conditions are when he goes to the range. This is one of a bunch of reasons results will vary considerably from published data.

One company that does use piezo equipment is Western Powders, the distributor of Accurate and Ramshot brands. Their website often lists piezo pressures with a bullets of the same weight but different brands in a particular cartridge. This can be very useful information.

The seating depth can also make pressure vary with the same load in the same rifle.It can be like adding a grain or more of powder just getting close the lands.So once again,pressure can vary greatly from published data and should only be used as a guide.The manual I really like is the Lee Modern Reloading.It list more powder/bullet combinations than any of my other manuals.A lot of what is listed is copied and pasted from other manuals,so it's like getting a bunch of manuals in one.And most of the velocities run fairly close too.

It still won't tell you anything about the pressures you'll get in your gun.

It is my understanding that if you use the same components that the manual shows and load to the same velocity (allowing for differences in barrel length) that your pressure will be pretty close to what the manual got. Otherwise no one would be able to recommend safe loads or even produce factory ammo that wasn't seriously under loaded for safety.

You won't know what that specific pressure is unless they report it, but can assume it is safe enough in your rifle.

That's why I'd appreciate seeing their pressure figures - either measured per load ala Lyman which is preferable or stating the actual MAP they used.


I see in the Nosler manual for the 6.5x55 they state their loads are safe in modern rifles - they do list some pretty impressive velocities - but then that leaves the milsurp or older rifle owner wondering where to stop in his individual firearm.

A few years ago, Ken Oehler shared a data set with me from one of his experiments. He was interested in my analysis of the data set.

The experiment involved instrumenting a barrel with two piezo transducers and two strain transducers. That allows some insightful analysis that probably isn't duplicated anywhere else.

The important findings are:

1. Data from the piezo system and data from a strain gauge system are equivalent and interchangeable for all practical purposes. The difference between the accuracy of the two comes down to the assumptions you make about the contribution of the brass case to confinement of the gas. In Ken's case, there was about a 2000 PSI difference in that assumption. Accounting for that, piezo data was, for all practical purposes, the same as strain data.

2. If you could make an absolutely perfect pressure measurement system, it would not improve the precision (test-retest error) of the measurement of MAP. The two main sources of variation in the measurement are cartridge to cartridge variation and random error in the measurement system. These two variations do not add linearly. The cartridge to cartridge variation is much greater than the measurement system variation, making the measurement system variation very hard to even detect. If you had a perfect measurement system, you could not make a more repeatable estimate than we now get.

MAP is not the actual upper specification on cartridge pressure. MAP is two standard errors below the actual upper specification, the Maximum Probable Lot Mean. However, the MAP is what we load to, because we are getting our data from a sample, and there is always uncertainty in results derived from samples. Setting the working upper limit down two standard errors provides a safety margin.

Yes, it is true that only The Almighty gets perfect, true information. The rest of us have to put up with estimates. Some of those estimates are good enough to be useful. It's still a measurement. But like all measurements, you have to recognize the limits of the measurement system.

The giant uncontrolled source of error in pressure measurements is that even SAAMI does not control barrel temperature, and barrel temperature is a very strong driver of peak pressure. When I'm doing precision testing, I strap a thermocouple to my barrel, just forward of the receiver, and do all my shots at the same barrel temperature. I also control the temperature of the ammunition. It's helpful to control ambient temperature, because it helps control barrel and ammo temperature, but that's not the temperature that counts.

Just for reference, one of the cartridges I tested, using ordinary components, was such that a 30* F change in barrel temperature produced the same change as a grain of powder. (IIRC, that was a Mosin.)

Finally, a strain gauge system can be calibrated and accurate. You believe your chronograph, don't you? So how was it calibrated? Did somebody whip out a can of NIST traceable FPS? Probably not, because such a thing does not exist. You know the distance between the sensors, and you know the frequency of the crystal clock in the chronograph. You count ticks of the clock, and you have transit time. You also have a formula that connects time and distance to speed, speed=distance/time. So you solve the formula and the speed result is as accurate and calibrated as your distance and time are.

To get accurate, calibrated pressure you need strain, chamber dimensions, and the properties of steel. All those are accurately obtainable to three significant digits, maybe four. Then you need the formula that connects those to pressure, and that's the Hoop Strain Equation. So, just like the chronograph, your measurement is as accurate and calibrated as your input variables.

Good stuff Denton.

Is the modulus of elasticity of the barrel steel really known to 3-4 significant figures without testing a specimen from the lot of steel the specific barrel was made from?

Related to the citation above for online data for Hodgdon's suite of powders:

Every year for the last decade or so Hodgdon has published a pretty complete manual of their loading data. It's updated annually. Generally the online site mirrors the manual, including velocity and pressure data for both suggested starting and max loads. I find working with printed data in the manual easier than the online text, especially when I have 5 or 6 sources of loads to compare for some cartridge. Too, it's a lot easier to scan a whole page of a couple hundred loads in the manual, compared with attempting to do the same with Hodgdon's online format.

The manual is sold out at Amazon and Midway, but it's available at Midsouth, Brownells, Natchez, Grafs, and other online suppliers, with some having it on sale.
--Bob
.

.

Denton,

Very interesting info from Ken's data, as well as the barrel-temp results--which I have seen myself in certain experiments.

But the problem with relatively low strain-gauge numbers definitely exists. It was first reported to me by two very experienced professionals in the business. Part of the problem, of course, is that some so-called professionals aren't very experienced.

For several years, one lab for a bullet company consistently reported that "improved" cartridges produced more velocity, at the same pressure, than larger cartridges of the same caliber. According to all the technicians at piezo labs I've discussed this with, they've never seen this in ANY cartridges of different powder capacity: In every instance, larger cartridges have always been capable of more velocity at the same pressure. There obviously isn't any reference ammo for wildcats, so the consistent error resulted from assuming the strain-gauge numbers were correct--and they were far from correct, as piezo testing later proved.

I have run also into chronographs that are not accurate, including some that started out accurate but became unreliable over time. Or at least that's the assumption made from direct comparison of their results with an Oehler 35P, by using both chronographs at the same time to record velocities of the same bullets.

Lot to lot variation of steel (of the same alloy) isn't so much of a problem. There are variations between alloys, though. The stainless alloys commonly used are a little different from the chrome-moly alloys, to the tune of 1,000 PSI or so. Apparently Kreiger uses an alloy that produces more difference than that. But, yeah, published modulus of elasticity data for specific types of steel is carried to three significant digits, and that's enough. You might get to four digits by the route you suggest.

In the end, chasing PSI is chasing the wrong rabbit anyway.

If you know PSI, the properties of steel, and the Hoop Strain Equation, you can design an ID and OD for the chamber that will not produce too much strain when the cartridge fires. Excess strain is what causes steel failure, and designers want to prevent that.

So what you really care about is the number of microstrains that the steel exhibits under stress. PSI is just a back door way to get there. Strain gauges measure strain directly. Why not take the direct route? IIRC, something like 500-550 microstrains, and you're good to go.

Of course you also want to operate below pressure levels that will cause brass to fail. Too bad it is so hard to get the wires to a gauge mounted on the brass case.

John...

I have no doubt that some users have produced erroneous results with strain gauge systems. Air bubbles in the adhesive under the strain gauge are one major possible source of error, for example. It isn't easy to get a perfect application, and air bubbles will give you low readings.

But Ken is a careful experimenter, and his strain and piezo numbers were extremely consistent with each other. The main issue was the offset used for the pressure contained by the brass case. Ken was a little frustrated because he was instructed to use a 7 KPSI offset and nobody could supply him with a satisfactory explanation of where that number came from. IIRC, using the Hoop Strain Equation and the material properties of brass, you get an offset closer to 5 KPSI. If I've got that right, a cartridge that reads 59 KPSI on a piezo setup will read 57 KPSI using the strain gauge approach.

At any rate what really matters is microstrains, not PSI, and gauges measure microstrains directly. If you keep the steel in the nice and easy strain region, it will not fail.

The magnitude of outer-surface hoop strains are trivial to measure with resistive type gauges (strain gauges). 400 to 1000 microstrain levels are a walk in the park for a sound and properly calibrated setup. The chamber is elastically loaded during firing and the outer surface strain gauge measures the resulting elastic deformation (hoop or circumferential typically). That data is easy to collect as long as the DAQ is up to the task.

Assuming the gauge was properly sized, bonded, located correctly (OD and ID measurements are known), and the gauge is truly oriented in the hoop direction, the fun really lies on how to relate the OD measured strain to the chamber pressure. The solid mechanics thick-walled pressure vessel solutions are simple and assume an ideal geometry defined by a constant ID and OD....not always the case when dealing with the chamber region of a firearm. The proper location for the gauge is not always ideal given geometric considerations associated with front ring receiver diameter as compared to the barrel shank diameter, barrel taper, etc. A quick FEA of a thick-walled pressure vessel having geometric discontinuities will quickly reveal that a strain gauge can lie within a stress gradient associated with these less than ideal thick-wall pressure vessels as rifle receivers/barrels often are. Strain gauges, especially 125 and 250 footprints (or larger) work best in uniform stress/strain fields..if they're over a gradient well then they're "averaging" the response underneath.

And then there's the issue of dealing with the cartridge brass contribution....brass having about half the elastic modulus as steel and a significantly lower yield strength. For the most part it is along for the ride (relies on the chamber for constraint) but when we're trying to nail down chamber pressure from a remote measured strain..devil in the details.