Home Forums Reloading Big Game Rifles Need help understanding pressure

Why is it that the WSM cartridges can be reloaded to like 62,000 and most of the other rounds are far below this? The actions are the same, are they not? Is the difference in the brass? Why do their primers not flatten? These questions may sound stupid to some but I need to learn about this. Thanks Vernonp

One thing you have to pay attention to is the unit of measurement, as most new cartridges have data shown in PSI (pounds per square inch), while older cartridges often have data that is shown in CUP (copper units of pressure). PSI and CUP are not the same, and this is one reason for the variation.

Another major reason for the variation among various calibers is that some cartridges were origininally designed roughly a century ago and chambered in rifles of that era. The strength of the old rifles often isn't equal to the strength of the newer rifles. Although the new rifles are capable of handling somewhat higher pressure, there are many of the old rifles still in use. Because of the old rifles still being used, ammo manufacturers and reloading component manufacturers keep using the original pressure limit. This is the main reason the newly designed cartridges are operating at higher pressures, because all the rifles built to shoot them are rifles that have been made in modern times and are capable of handling the higher pressures.

Although the brass does have an effect on how much pressure you can get away with, the brass isn't the real reason why the new cartridges operate at higher pressures; see what I said above for the reason.

Unless there is a headspace problem, flattened primers in a rifle cartridge typically will not occur until you're well above 65,000 PSI, which is why most reloaders are wise enough to back down the load if they get a flattened primer.

That's funny, because I was wondering the exact same thing today <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />. I asked my science teacher, and he didn't even know what CUP stood for. Is there away to convert CUP to PSI?

270jrk

[quote]That's funny, because I was wondering the exact same thing today <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />. I asked my science teacher, and he didn't even know what CUP stood for. Is there away to convert CUP to PSI?

CUP is a measurement of how much deformation of a brass pellet there is when it is subjected to pressure. PSI is how many pounds of force per Square inch the pressure measures. The last I knew there was no accepted formula for converting CUP to PSI. I have seen a formula posted some where that claimed to do it but, I don`t believe it was proven with the higher pressure rounds.
The pressures a cartridge gives is set by SAMMI usually with reguards to the strenght of guns chambered for it. If a given rifle can handle 65000 psi with a cartridge rated at that pressure there is no reason it can`t handle a cartridge with the same pressure as long as the brass can.

The WSM's are not the only ones that have higher limits for PSI; the 308Win and the 270Win are others.

Cup and PSI are different measuring methods, that attempt to measure the same thing, but don't. Cup is a more indirect measurement -- kinda like measuring distance you have traveled by how much gas your truck has used. It's pretty accurate on average, but you there are circumstances where you can be off considerably. Like if you drive from Sacramento to the top of Donner Pass (gaining 8,000 foot). You would overestimate the distance if you used only gas consumption to calculate it.

When the industry started measuring some loads with Piezo gauges (especially with very fast powders), several loads read WAY higher than anyone expected: the copper crusher simply did not have TIME to deform on those loads. HTH, Dutch.

What the others said, with one addition.

When piezo-electric equipment became standard in the business, it was found that some cartridges showed wider pressure swings (from low to high) than had been measureable with copper crushers.

SAAMI (Sporting Arms & Manufacturers Institute) pressure limits are not based only on averages of a number of rounds, but the highest pressures shown in a string of rounds. This is why the .243 Winchester and 7mm Remington Magnum, for instance, have lower SAAMI pressure limits than the 6mm Remington and .270 Winchester. They tend to show a slightly wider range of pressures in a given string of shots when tested with piezo equipment.

The short-fat magnums in general show lower pressure swings than many other cartridges, hence can be loaded up a little compared to cartridges that exhibit wider pressure swings.

MD

Simple answer: You can drop a 30 06 into a hundred year old rifle, but you can't do the same with a WSM, unless you have deep pockets and a patient gunsmith. If you really think about it, the WSM and the Remington counterparts are bullet technology catching up to rifle technoligy; newer and better metals and machining.

I'm a little late plugging into this, but maybe it will be worthwhile to explore this again.

In the U. S., cartridge pressure (the average peak) is established by the cartridge designer/manufacturer and sent to SAAMI (Sporting Arms and Ammunition Manufactuers Institute) for codification. Within certain limits (no brass case in excess of 65,000 psi for example), they will generally accept the cartridge designer's specifications, and that becomes the industry standard.

To answer your original question, though, is there a technical reason why the '06 has a SAAMI peak pressure rating of 50,000 CUP/60,000 psi while the 270 Winchester has 52,000 CUP/65,000 psi. the 25-06 Remington has 53,000 CUP/63,000 psi, and the 7mm RemMag has 52,000 CUP/61,000 psi? No, not really other than what the designer/manufacturer considered good practice at the time.

There are, of course, peak pressure limits established because of action design. The 30-30 Winchester (Winchester M94), 300 Savage (Savage M99), 35 Remington (Remington M8 semiauto), and .45-70 (Springfield 1873 "Trap Door") come to mind. These were designed for actions that were weak compared to double-lug, front-locking bolt actions like the '98 Mauser, '03 Springfield, Winchester M54 and M70, and so on.

This is by no means an exhaustive statement on the subject, but I think I have related the essence of it.

OK Shooter good to see you posting.
Is it also possible the max average pressure could be a result of the cartridges behavior with powders being used to develope it in regards to pressure ES? I have heard the 243 Win and 7Rem Mag pressures were lowered because of spikes in pressure causeing acceptible avg pressures but exceding the max peak allowed. I`ve wondered about this when noteing newer cartridges (less then 50yrs old) with pressure limits lower then say 62-65000psi. The 223 with 55000psi and the 260 Rem at 60000 psi SAAMI avg max for example. It can`t be brass or rifles holding the psi down and I doubt the manufactures want to "down load" any new round.

Ol' Joe -- Yeah, sometimes it can be a mystery. With respect to the 223 Remington, I understand it had to do with the M16/AR15 action opening too quickly. I know for a fact that there have been no mysterious pressure spikes with any propellant and bullet weight that I've tested in bolt actions.

The 260 Remington, I think, was kept at the same pressure level of its parent .308 Winchester cartridge. I don't have any experience with the 260 Remington, but I suspect it was simply a Remington decision to do that but have no clue as to their rationale.

The other aspect, I think, is the old crusher way of doing things. For example, the .308 Winchester is SAAMI rated at 52,000 CUP; and when Remington brought out the 7mm-08, the submitted it at the same CUP rating as the parent case. It just so happens, though, that the .308 Winchester develops 60,000 psi at this 52,000 CUP level while the 7mm-08 develops 57,500 psi.

A similar thing I think happened when they introduced the 7mm RemMag back in the early '60s. It appears as though they tried for a pressure similar to the 270 Winchester (52,000 CUP/65,000 psi), but the 7mm RemMag at 52,000 CUP develops 61,000 psi. Then, again, we have the 8mm RemMag, and I suspect they tried for the same pressure level as the 300/338 WinMag (54,000 CUP) but exceeded that slightly since the 8mm RemMag at 54,000 CUP develops 65,000 psi.

The crusher method of pressure testing has been a thorn in the side of ballisticians forever and a day! And in the final analysis it doesn't make a lot of sense either! Why, for example, does the 243 Winchester at 52,000 CUP develop 60,000 psi while the 6mm Remington at 52,000 CUP develop 65,000 psi? I haven't even tried to figure that one out and would consider it a waste of time to do so.

These considerations do, however, bring up another situation. If we have these kind of divergences in laboratory setups with carefully formulated and controlled copper slugs, how in the world is that we can expect case head expansion in average run-of-the-mill brass cases to tell us anything quantitatively? Qualitatively, of course, it can tell us when the brass is reaching or exceeding its elastic limits, but so will an expanded primer pocket.