Don't understand pressure - need expert help
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  1. #1
    Tinhorn
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    Don't understand pressure - need expert help

    I see that pressure is different for different calibers as opposed to different chambers, but why?? How is the pressure determined and why by caliber? Shouldn't the barrel and chamber basically the same strength for all cartridges...unless its an old one? Example...If it is determined on chamber strength, isn't my Marlin 1895 as strong or even stronger than my AR-15 chamber ??Yet the Ar-15 can be loaded to much higher pressures. Why?? Is also based on brass strength or something??
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  2. #2
    dla
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    Don't understand pressure - need expert help

    I not an expert, but I think I can shed some light on this.

    The strength of a chamber is dependent on the thickness of the walls and the material the walls are made of. Usually this "strength" I'm referring to is resistance to becoming plastic and permanently deforming (i.e. ringing, bulging, exploding).

    Rifles vary in construction, depending on the intended use and the SAAMI or NATO operating limits for a particular cartridge. The SAAMI Maximum Average Pressure, which is the average of the peak pressures measured on numerous rifles not the average peak in the barrel, for the 45-70 is 28Kpsi. The Marlin 1895 is designed to safely handle 28Kpsi and may in fact provide an over-load safety cushion of 2X.

    It is difficult to exceed the safety cushion with normal loading practiices with the 45-70 in the 1895. However your AR can be destroyed by jamming the bullet into the lands. I don't think the AR design has a 2X saftey cushion.

  3. #3
    Tinhorn
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    Don't understand pressure - need expert help

    But the 45-70 could be loaded in a bolt action of much higher strength. Nonetheless, the SAAMI pressure would be the same for this cartridge and it is considerably lower than other cartridges. Also, I see the Garrett makes heavy loads that do exceed SAAMI...+P. Or do they? What is the rationale? How does SAAMI determine the pressure?
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  5. #4
    Distinguished Master
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    Don't understand pressure - need expert help

    I am not going to say a whole lot on this except as DLA said SAAMI is 28,000 for the 45-70 yet the .450 Marlin is 43,500 PSI and then in the same action we have the new .480 Ruger and the mighty .475 Linebaugh with a Hodgdons max as 55,000 PSI and all in the same 1895 action.Some and most accept 40,000 CUP as a safe max for the modern 45-70 and then there are those experts that say more and some people say less.

    I think 40,000 CUP is a good max as recoils really jumps past that but like others ask,why are they loading 55,000 PSI cartridges in the exact same action?Legally I suppose it is SAAMI pressure but if you reload any of them your on your own.The action fails by not locking or coming unlocked before the barrel does which tells us the Marlin is one tuff customer.

    It was like 1980 before they came out of the closet with 40,000 with little or know data at that time for it but since a few and very few give data to that range.

    Good luck............Jayco.
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  6. #5
    Tinhorn
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    Don't understand pressure - need expert help

    Precisely my question Jaycocreek. Same action = different SAAMIs. Why? Why ? Why? How is SAAMI determined
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  7. #6
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    Don't understand pressure - need expert help

    Quote Originally Posted by widmn
    Precisely my question Jaycocreek. Same action = different SAAMIs. Why? Why ? Why? How is SAAMI determined
    One reason is the older 45-70's around that some nut case would put a 40+PSI/CUP whatever round into and blow his self up and lawsuits would fly.You can't do that with the .450 Marlin-.480 Ruger and .475. Different cartridges and brass and wont fit even so the same action.

    Jayco.

    I'll let some one else answer the SAAMI question as I am not a huge fan and don't want to ruffle feathers.
    Howdy Boys.

  8. #7
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    Don't understand pressure - need expert help

    Try this sight for official SAAMI talk.
    http://www.hpwhite.com/101-00.pdf

    Jayco.
    Howdy Boys.

  9. #8
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    Don't understand pressure - need expert help

    Hey widnm. As jayco pointed out in the case of the Marlin the action is the same one used for several other cartridges that have saami pressures that are much higher. I as well as several other fellers have tested the Marlin 45-70s and found that they are capible of much higher than 28,000cup without causing harm to the gun. There has been ongoing lobbying for a +p designation for Marlin nad Ruger level loads but so far it is to no avail. I don't even hesitate to stick 50,000cup loads in my Ruger. It has had hundreds of +p loads down the tube and still drives tacks. reflex264


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  10. #9
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    Don't understand pressure - need expert help

    Quote Originally Posted by reflex264
    Hey widnm. As jayco pointed out in the case of the Marlin the action is the same one used for several other cartridges that have saami pressures that are much higher. I as well as several other fellers have tested the Marlin 45-70s and found that they are capible of much higher than 28,000cup without causing harm to the gun. There has been ongoing lobbying for a +p designation for Marlin nad Ruger level loads but so far it is to no avail. I don't even hesitate to stick 50,000cup loads in my Ruger. It has had hundreds of +p loads down the tube and still drives tacks. reflex264
    Hey Reflex- I read an article today by SAAMI saying +P loads are not SAAMI spec't now nor ever.True or not I have know idea butt the 40,000 thing came out in 1980 and now were at 55,000 PSI in the same action.So So confused as always and a bigger hole in the barrel.

    Dr. Phil time for me like old Bestlever.

    Jayco.
    Howdy Boys.

  11. #10
    Tinhorn
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    Don't understand pressure - need expert help

    No, that's tyrue. +P and +P+ loads have always been over SAAMI specs. I guess that the problem is that no one except Garret has made an "official +P load for the 45-70 yet and designated it as such. At least not to my knowledge. I believe that the Garret loads are in excess of SAAMI and they are designated as +P ??
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