Home Forums Hunting & Shooting Ask The Gunwriters 'Hidden' Cartridge Characteristics - SAAMI Pressure Specifications

You forget a little thing called "Marketing Hype". Anyting for a sales advantage, whether real or not.


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Given equivalent brass & actions, they should all be rated at the same maximum PSI.........no other way to interpret it IMHO.

Unless you subscribe to the theory that there are some "maverick" cartridges out there that have some extraordinary pressure excursions.....this also assumes equivalent throating, bullet seating, etc.

Pressure is pressure; it doesn't know what the cartridge is that produced it.

MM

The same with a 30-06? How about a 7x57? Or a 30-30 in a bolt gun?.


Casey

Succinctly put, Ken.

As for the brass, I believe the SAAMI piezo standard tries to correct for the effects of the brass. How the company which developed the combination of cartridge and rifle decides what is acceptable is not standardized.

Thanks for the correction. It didn't look right to me when I typed it, admittedly.

As others have said since, a 5 Kpsi difference is essentially no difference, and the major value of some +P (and "Magnum") in an appellation is mostly the marketing people cashing in on the belief that more is better. Even if you only call it more.

The Versatile .280

By Jim Carmichel

The .280�s prospects were further reduced when word got around that Remington purposely �loaded it down� so the gun could be safely chambered for its recently introduced Model 740 autoloading rifle. This curse dogged the .280 for years and is occasionally repeated even today.

Several years ago I became intrigued by this chapter in the .280�s history and checked out the �loaded down� rumor with a few of the older heads at Remington. As with many rumors from the shooting industry, there�s a grain of truth to this one, but the real facts I gleaned tell us a lot more.

Unlike earlier autoloaders such as Remington�s M81, which were limited to mild cartridges such as the .30 and .35 Remington, the M740, which was introduced in 1955, was designed for rip-snorting calibers such as the .30/06. It was a successful rifle and would have been even more so had it been chambered for the .270 Winchester, but it wasn�t. And the word going around at the time was that the M740 couldn�t handle the 270�s pressures. And here�s where the strange saga of the .280 gets particularly interesting.

Not counting some of Remington�s interoffice politics, personal opinions and jealousies regarding the .270 during the 1950s, I learned that the M740 and .270 actually did not make a good match. Not necessarily because of the .270�s high pressures, but because the M740 tended to be finicky about what it was fed, its gas-operated system being reliable only when adjusted to rather specific pressure levels.

The .270 loads of the day, I was told, tended to develop varying pressure levels, which in turn could have resulted in the M740�s erratic operation. Thus the .280 was not so much �loaded down� as loaded to specific pressures compatible with the M740. Apparently, it isn�t often noticed that the M740 was also chambered for Remington�s new .244, a hot round that, like the .270, generated pressures over 50,000 PSI. In 1960, when the M742 replaced the M740, it too was catalogued sans the .270.

The full article is here:
http://www.outdoorlife.com/outdoor/shooting/article/0,19912,1153256,00.html
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I thought some might find this interesting.

Cheers!
-Bob F.

Is that because the same (roughly) amount of burn/pressure is concentrated on an ever so slightly smaller surface area?[/quote]

Nope. Pressure limits are determined by the chamber head dimensions, not the throat.

Also A .270 throat would have thicker walls than a .280 so pressure is not a consideration.

Throat erosion is mostly a function of sustained temperatures not pressures.

Or, to put it another way.
When you stuff the same volume of hot gas through a smaller hole, your gas erosion rate increases. Extreame example would be to compair the erosion rates of a 35 whelen to a 25-06.

However, even the .25-06 doesn't have the 'rep' for 'erosion'. I imagine that the caliber difference alone would account for a negligible difference between .270 and .280. I've read that the .243 does have a bit of a rep for 'accelerated' erosion. I wonder if that's simply the relationship between the volume of powder and bore size, or if there are more complicated dynamics involved. For instance AFAIK, the 6mm Rem doesn't have the same rep for erosion as the .243, even though it has a (slightly) greater powder capacity, higher velocity etc. Do the 'stubby' cases result, to some extent, in higher peak pressures and temperatures due to the 'powder column' being shorter and therefore burning more quickly?

No, the stubby cases do not burn the powder more quickly. Mostly they burn it more consistently, with less deviation between higher and lower pressures in the same string.

Nobody can really say for sure why the .243 tends to erodes throats quicker than the 6mm, but it is suspected that the long, sloping shoulder and short neck have a lot to do with it. When the same case is necked up, even to .260, a lot of this goes away, because the neck gets longer.

And yes, there are indeed some cartridges that show wider pressure variations than others, on average. In general the short-fat magnums show less variation than longer rounds of about the same powder capacity.

If you look at the SAAMI "average" pressure for any round, you'll find some that have only been chambered in so-called "modern bolt-action" rifles that have a noticeably lower average pressure than others. These are generally rounds that have shown a little wider variation in pressure. This is nothing that the average handloaded will be able to "see" in looking a fired primers or anything else, but something that has been observed in many pressure labs. Hence the SAAMI consensus to keep average pressure a little lower.

You will also not be able to see much difference in the pressure curves of various powders in a certain round. If a powder is at all appropriate to that round (whether a canistered powder available to handloaders, or a special powder available only to ammo makers), the pressure curve will not vary all that much.

JB

I think there is some theory as to cause of throat erosion based somewhat on neck length and shoulder angle as you have said. That has to do with the turbulent vortices that form after the bullet leaves the case. These are either centered in the throat or somehwere inside the case neck. I believe the Weatherby double agive (ogee) shoulder/neck was an attempt to reduce throat errosion by containing these vortices some where other than in the throat of the chamber.

And I agree about the short cases, I hesitate to use the term short/fat cases but a cylindrical (no taper) case with a width to height ratio such as the SM case does have a more uniform burn and a more uniform pressure development curve than a long tapered case such as the H&H style case. I suspect because of this turbulence. (less opposing forces internally) I cannot attest to the actual advantages or disadvantages of either, but ES of pressure is less with the short fat cases. I would also guess throat erosion would also be reduced but by some amount given comparable neck length, caliber, etc.

When looking at pressure curves the time base must be turned down to see the differences and the time that a curve is above a specified level can vary no more than 150-300 micro seconds, we normally look at these for a duration of a few (4-10) milli seconds. The propellant used in the light mag ammo (30-06)is very slow, about like RL-22/H4831 etc, but with a higher percentage of NG than most other double base powders (more energy per grain) and the curve is "up" for a longer duration than 4831 or RL-22 by about 200-250 microseconds. This gives us a greater time duration with the pressure acting on the bullet yet still keep the peak pressure at or below SAAMI pressure. It is force exerted per unit of time that increases the accelleration of the bullet. We can increase the pressure to increase the velocity, or keep it the same and increase the time the pressure is applied. Certainly the slow burners (RL-25)have a longer time duration, above a certain pressure line, than fast (RL-15) powders.

Yes I think you're right about the light mag/high energy stuff in the auto loaders. It is more a function of pressure at the port and with these high energy slow burning powders that port pressure is still too high when the bullet passes the port and can damage the mechanism.

Always a good idea to stay in design limits but it does seem logical that in a modern rifle with all brass the same in material and thickness, etc such as all 30-06 based rounds, that we can load them all to the same pressure. I think most of us do as handloaders. Certainly the ancient 7x57 in a modern rifle with good brass can be loaded safely to 308 pressures. It's speced pressure is 45kpsi and the 308 is 60kpsi.

Same theory of light mag powders applies to the N540/550 that I burn so much of in my 223s????

The article still fails to answer the question as to why the Remington 244 was loaded to 65,000 psi and chambered for the
740. It was not loaded to a specific chamber pressure to fit the rifle as Jim states. Also in 1981 the model 7400 was added to the Remington lineup with the 270 and 7mm mag added to it's list chambering with the 280 still loaded down and the 7mm express coming onto the scene a few years previous. Have to wonder at that opportune time why Remington did not raise pressures in it to match 270 velocities.

Remington has always baffled me when it comes to their 280.

http://www.remington.com/library/history/firearm_models/