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Loads vs SAAMI max pressure

Discussion in 'Handloading and Reloading' started by LawBot5000, May 13, 2007.

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  1. LawBot5000

    LawBot5000 Member

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    Why are the max pressures all below SAAMI max (quite a bit below in some cases) when guns are supposedly overbuilt these days, especially in comparison to the strength of guns in the early part of the 20th century.

    And how far have the SAAMI specs been reduced in recent years? I remember that 7mm remington was a stronger round back when it was created.
     
  2. steveno

    steveno Member

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    the pressure measuring equipment is alot better. a lot of loads were just checked by looking at the primer condition after firing and hot easy the bolt lifted. neither of which is a good indicator of pressure
     
  3. LawBot5000

    LawBot5000 Member

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    I just assumed it was an ass covering measure because some people had rifles that had flaws leading to failure.
     
  4. steveno

    steveno Member

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    there might be a little CYA but some of the loads way back when were very hot
     
  5. Art Eatman

    Art Eatman Administrator Staff Member

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    For one thing the measuiring units have changed. "Way back then", it was psi--pounds per square inch. They changed to CUP--copper units of pressure.\

    As near as I can tell, one CUP is a bit more than one psi. In raw numbers, then the pressure in terms of CUP appears less than in the old data books that used psi.

    The limiting factor, though, is blowing primers. Hard to actually blow up any rifle. Either the case head lets go, in a claw-feed action, or the primer blows out.

    Primer blow-out is the most common Oops! Not harmful, but certainly not Good News, either...

    Art
     
  6. mc223

    mc223 Member

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    Last edited: May 13, 2007
  7. LawBot5000

    LawBot5000 Member

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    I'm not an expert yet, but I could have sworn I heard stories of people cracking rifles open with hot loads.
     
  8. Ol` Joe

    Ol` Joe Member

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    Acually manuals used to list pressure in PSI when they were measuring it with copper crushers. They started calling it CUP in the late 70s early 80s when newer measuring systems, Piezo and strain gage, came along and really did show pressure in PSI. The old CUP/PSI rateing was misleading and is not the same as todays rateing.
    One other thing to consider is the SAAMI max we talk of is just one of two rateings, SAAMI MAX AVERAGE and SAAMI MAX ALLOWED. We see the MAX AVERAGE and think of it as an absolute figure when it is a average of pressure found. The pressures could have been ranging from 4K-5K psi over to the same under, and a SAAMI max of 60K MAX AVG for a cartridge with a MAX ALLOWED of say for instance 63K may mean they had to stop at 58K with the load to stay in safe pressure ranges. This is the result of more accurate measuring systems showing what they are really getting and not a estimate based on the deformation of a copper plug measured by a tech with a caliper.
    I have read of a couple of cartridges that had the max pressure rateing dropped due to the findings of pressure spikes with the new equipment, or to the old firearms that can`t be trusted to take the high psi anymore. The 7mm Remington mag and 243 Win I believe are two guilty of the 1st example that have had their max psi lowered. I can`t say for sure this is true but no ammo manufacture wants to drop their ammo pressures/velocity and have other companies loading theirs hotter. It just isn`t good for business. There has to be a reason that makes it prudent, and too all manufactures have done it when you hear of these cases not just one trying to please their legal people.

    BTW
    The 7mm mag is rated at 52,000 CUP or 61,000 PSI which isn`t very low to be honest. The highest pressure of any cartridge SAAMI backs is 65,000 psi.
     
  9. Steve C

    Steve C Member

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    I assume you are asking why the load manuals don't give you a load that reaches maximum SAAMI pressure?

    There are several reasons. The primary being that due to variability of components, lot to lot and one manufacturer to another, your resulting pressures and ammunition performance will vary from what the manual indicates for a given charge weight for a given weight bullet in a given make of brass using a given primer brand, etc.

    Hand loaders do not usually have the the expensive and sophisticated equipment used by the ammunition manufacturers to produce exacting selected pressures being developed by their ammo. Hand loaders will often see pressure signs well before they reach the maximum charge listed in the loading manual indicating pressures well beyond SAAMI maximum standards due to this variability.

    This is why reloading manuals are GUIDES not COOKBOOKS and you DEVELOP loads not just pick a RECIPE.

    Another reason is that pressure and the amount of powder used is not a linear product IE. if X grains of powder gives you 15,000 psi then 2X will most likely NOT give you 30,000 psi but something far greater. The results of small increases in powder at higher pressure levels can result in significant increases in pressures or produce erratic results.

    SAAMI specs have changed measurement, the older units where CUP or Copper Units of Pressure or at least thats what all my older manuals (1970 and later) shows while the current pressure units used are PSI (lbs. per square inch). Copper Units are an inexact non linear scale of measuring pressure and there's no direct correlation to PSI or formula to translate CUP's to PSI's.

    The older copper crush method of measuring pressures tended to get an average pressure and missed some of the peak pressures. While the SAAMI standards haven't really changed other than adding some categories like +P the current more modern method of measuring pressure is more exacting and detect pressure spikes that the old method didn't detect. This has resulted in the lowering of some charge recommendations in the reloading manuals for some powders.

    For the most part ammunition ballistics are the same now as they where 35 years ago if you pay attention to the method of testing. Old factory ballistic data used to be taken from long test barrels with a fixed breach. This would give a resulting velocity much higher than actual from a typical 4" handgun, esp. in revolvers where you had some efficiency lost due to gas leaking from the barrel/cylinder gap. For example a .38 spl +p load a 158gr lead bullet leaves my 4" S&W M66 at 879 fps and 1,100 fps from my 18" Timberwolf carbine. A .357 mag load that's 1,232 fps from a 4-5/8" Ruger BH runs 1,560 fps from the carbine.
     
  10. mc223

    mc223 Member

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    Please see the link in my post above. May be enlightening.
     
  11. LawBot5000

    LawBot5000 Member

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    Got it. Thanks for the awesome amount of info guys. Sounds just like tuning a car.
     
  12. Ben Shepherd

    Ben Shepherd Member

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    Just remember, if you're going to push things:

    Go slow. Go careful. DON'T do it without a chrono.
     
  13. LawBot5000

    LawBot5000 Member

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    I agree, but a chrono wont be much help to me since I am just aiming to make cleaner 45 ACP subsonics. If I get the heavier (260, 300gr) 45 LC slugs to go transsonic in a 45 carbine without damaging the brass, that would be dreamy.
     
  14. Clark

    Clark Member

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    The speed limit may be 55 mph, but the car can go faster....if you know what you are doing.
     
  15. CMcDermott

    CMcDermott Member

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    This article may be good mathematics, but it's lousy engineering and will get you into trouble fast.
    Look at the graph of SAAMI CUP to PSI, and it is obvious that many points aren't close at all to the line of correlation. That 7% error is an AVERAGE, some cartridges are much farther off high, some farther off low, some are on the line. Trying to use a single conversion equation will lead to bad results.
    Suppose you have an old load that the manual says produces 50,000 CUP. You know your rifle will work fine with a 60,000 PSI level. So you use his equation to convert 50000 CUP to get 57891 PSI. But now you need to throw in the error factor of 7%, so the result is really a range from 53839 PSI to 61943 PSI. So what do you do to develope your 60,000 PSI load, add powder because the pressure level could be only 54K PSI, or subtract some powder because the pressure level could be over 61K psi? For those of use that had a college statistics course - do you ever remember using a confidence level below 95%? Do you remember that most of your work was with confidence levels of 99% and 99.5%? Is that 7% error level really close enough for use in the real world - I think NOT!

    You can do things mathematically that just don't make sense in the real world. Listen to the professional ballisticians when they say you can't do a general correlation of CUP and PSI pressure levels.
     
  16. CZ57

    CZ57 member

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    The test methods vary beyond the unit of measurement. They also vary in the type of maximum pressure being determined. I've talked to a few ballisticians about this and most agree that the Piezoelectric measuring system is more accurate than CUP, but mainly because of the advance in technology and the introduction of the piezoelectric transducer into ballistic research. The author of the formula from the link provided, also mentioned Commissione Internationale Permanente, or CIP. Many ballisticians feel that the European CIP system is superior to the SAAMI system that looks at the maximum average peak chamber pressure. CIP values are in PSI, and BARs, and the PSI values are very similar, but not identical, to values collected by the Copper Crusher method. This stands to reason because the transducer in the CIP system is looking for a similar pressure condition to that of the Copper Crusher. This is not to say that the CUP system is inaccurate. It's been in use about 125 years, and many are still using it, and particularly with high pressure loads, with satisfaction that it is accurate. In a very few cases, some powders will register higher values in CUP than they do in PSI, but these are very isolated cases and not likely to be reproduced by your average handloader in the case of using Herco in a .30-06 load for anything other than very low pressure cast or plinking loads. A good explanation of this can be found in the Lyman manual in the chapter titled: "A Limited Comparison of the Crusher and Piezo Systems."

    I believe that what ballisticians say is that there is No Direct Mathematical correlation, as in the case of deriving a formula for direct conversion. This does not mean that they can't be compared with fairly high accuracy. As far as lousy engineering, I've seen plenty by some that were totally incapable of design and in many cases, evaluating their own data. The Corps of Engineers comes to mind, and I've had direct experience with them, and however they go about selecting their engineers needs to be re-examined. I'll spare you the story.

    Many of you may not be familiar with Graphic Geometry, and you may not want to take the time to do this anyway. But, using known values for minimum and maximum that are equal, but expressed in the two different standards of measurement, you can graph the values and get very close. Under 200 PSI accuracy, anyway. To establish a base line you need to look at cartridges that are very low in pressure with values given in both CUP and PSI. The maximum is easy to come by since it's been around since 1925 and the .270 Winchester @ 65,000 PSI that is equal to 54,000 CUP. The same pressure used by the newer short fat magnums. At lower values the numbers are much closer, obviously, than they are as they reach maximum. Hence, no simple multiplication of one point something or other. Nor, some mathematical permutation that corrects the formula as values increase. You establish a single height for the graph with some form of uniform increment such as various forms of graphing/sketch paper. The angle of the PSI line is more severe than it's CUP counterpart. You can graph in such a way as to get divisions of 100 units of value for both PSI and CUP and then a linear comparison is possible.;)
     
  17. Clark

    Clark Member

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    CAUTION: The following post includes loading data beyond currently published maximums for this cartridge. USE AT YOUR OWN RISK. Neither the writer, The High Road, nor the staff of THR assume any liability for any damage or injury resulting from use of this information.


    22-250 Remington 65000 53000 CUP
    243 Winchester 60000 52000 CUP
    6mm Remington 65000 52000 CUP
    250 Savage 45000 CUP
    257 Roberts 54000 45000 CUP
    25-06 Remington 63000 53000 CUP
    260 Remington 60000
    6.5x55mm Swedish 51000 46000 CUP
    270 Winchester 65000 52000 CUP
    7mm-08 Remington 61000 52000 CUP
    7x57 Mauser 51000 46000 CUP
    280 Remington Express 60000
    300 Savage 47000 46000 CUP
    307 Winchester 52000 CUP
    308 Winchester 62000 52000 CUP
    30-06 Springfield 60000 50000 CUP
    8mm Mauser 35000 37000 CUP
    8mm-06 50000 CUP (Speer)
    338-06 53000 CUP
    356 Winchester 52000 CUP
    358 Winchester 52000 CUP
    35 Whelen 52000 CUP

    If you look at those examples where the pressure is near the limit [65,000 psi] of the 1892 7mm Mauser rimless case head design, you see that the psi is 10,000, 11,000, or 12,000 higher than the cup figure for SAAMI registration.


    One has to ask, "Who cares?", because there are a couple problems:
    1) All the Mauser case head cartridges are capable of the same pressure before the primer pocket grows, and yet are registered for different pressures.
    2) For the handloader in a strong rifle, it does not matter at what pressure the primer pocket grows, the goal is to work up to the threshold load and then reduce the load by a safety margin to protect the primer pocket, ala Vernon Speer' 6% in 1956.
     
  18. CZ57

    CZ57 member

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    Clark, in some cases the Cup rating has been altered without the equivelant change in PSI. The .308 hasn't changed and the 62,000 PSI and 52,000 CUP ratings are equivelant, or very close. In the case of others where the PSI rating is unaltered, but the CUP rating has lowered, it's not much of an issue because the load data in PSI won't exceed the lowered value in CUP. Personally, I could live without the ambiguity that SAAMI provides when they start tinkering with long established pressure standards.;)
     
  19. trueblue1776

    trueblue1776 Member

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    Good accuracy is all I care about, I'm fine with a minimum load as long as it will hit what I'm shooting at and cycle an action (if loaded for semi). I can't afford to replace any of my guns (or my face) so I don't get creative with hand loads and I double check ALL sources of data. Plus who wants to buy more powder than they must?
     
  20. CZ57

    CZ57 member

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    This about more than cliches. For instance, you hit the lottery for a permit to hunt Elk in Colorado. You going to use your minimum accuracy load, or, are you going to develop a safe accurate load to meet the possible challenges of the hunt? That's why we have accepted maximums: they let us know the safe limit that we can load to if necessary, and sometimes, it is.;)
     
  21. trueblue1776

    trueblue1776 Member

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    I forgot to add that I'm really really cheap.
     
  22. CZ57

    CZ57 member

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    Ain't we all!;)
     
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