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Thread: Formula for chamber preasure..?

  1. #1

    Formula for chamber preasure..?

    Hey guys,
    Im sure this is a newbi question, but....lol. Everyone is always talking about the preasure levels that SAMMI gives us for each caliber. That i understand perfectly. But when it comes to reloading your own ammo such as 45 Colt, how do you know what the chamber preasure will be for the powder/bullet combo that you are putting together. I see on reloading charts they will have bullet weight, type of powder, powder weight, primer, case, etc..., but dont list preasures. So how do we know?

    Thanks,
    Blacky

  2. #2
    Boolit Master GhostHawk's Avatar
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    You don't.

    Manuals, what I consider "good" sources list starting and max loads, list velocity, and pressure.

    Data that does not come with such information simply is not trusted as much.

    The other answer is you don't, but experience has shown this to be a safe load so you don't worry about it.

    For example in .45 colt I have 2 molds, both 230 gr but one is a truncated cone, other is a round nose.

    I look in the manual, compare, both very similar velocity's, pressures. The round nose somehow hangs on to a bit more energy. But they have similar loadings. 6.5 grains of Red Dot is in the moderate portion.

    If you are not getting your data from a source that includes velocity and pressure. Or unless you have one of those pressure testing setup's like Mr Larry has, you are not going to know.

    But you can make an educated guess.

  3. #3
    Boolit Buddy am44mag's Avatar
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    Some manuals do list pressures. I believe the Lyman manual does (49th ed). Hodgdon's online reloading data system also lists pressure.

    http://www.hodgdonreloading.com/

    As for calculating it yourself, that's easier said than done. There's not a simple formula like y=mx+b to just punch numbers into. There's a lot a variables. I'll tell you right now that thermodynamics is not fun to learn, and it's definitely not easy to understand. Someone more knowledgeable than me might confirm this, but I believe that the big companies use a special device to measure the chamber pressure, not a formula.

    The best you can do is look at published pressures and guesstimate what yours is. I've never found it particularly important to know as long as I am under the SAMMI listed maxes. Just work up your loads starting at min loads and watch out for pressure signs such as blown/pierced primers, backed out or flattened primers, sticky extraction of the cartridge from the gun, and pressure rings. This article covers it.

    http://www.mssblog.com/2018/03/08/re...ressure-signs/

    The two most common signs I've run into are flattened or pierced primers.
    ______________________________________________
    Aaron

  4. #4
    Quote Originally Posted by am44mag View Post
    Some manuals do list pressures. I believe the Lyman manual does (49th ed). Hodgdon's online reloading data system also lists pressure.

    http://www.hodgdonreloading.com/

    As for calculating it yourself, that's easier said than done. There's not a simple formula like y=mx+b to just punch numbers into. There's a lot a variables. I'll tell you right now that thermodynamics is not fun to learn, and it's definitely not easy to understand. Someone more knowledgeable than me might confirm this, but I believe that the big companies use a special device to measure the chamber pressure, not a formula.

    The best you can do is look at published pressures and guesstimate what yours is. I've never found it particularly important to know as long as I am under the SAMMI listed maxes. Just work up your loads starting at min loads and watch out for pressure signs such as blown/pierced primers, backed out or flattened primers, sticky extraction of the cartridge from the gun, and pressure rings. This article covers it.

    http://www.mssblog.com/2018/03/08/re...ressure-signs/

    The two most common signs I've run into are flattened or pierced primers.
    Yes the Lyman manual does list the pressure. I'd suggest going to like cabelas or bass pro where they have several manuals. Look at a cpl and then decide what you feel is best for you. I say go there because you can look at them. Personally I'd buy it somewhere else.

    Sent from my XT1650 using Tapatalk

  5. #5
    Ok.. let me clarify my question just for the record. I would never use any reloading data without knowing preasures. What brought up the question, was last night i was on one of the websites looking at the "three tier" levels listed for the 45 Colt. Being 14,000, 20,000, and 32,000. On each of the three charts they had all the info listed for different powders and such, but NOT indivual preasures. You just know for example that anything listed on chart #2 was below 20,000 PSI. I have one of those inquiring type minds, so i wondered if there was a way to figure the individual preasures.

  6. #6
    Gotcha

    Sent from my XT1650 using Tapatalk

  7. #7
    Boolit Master


    Larry Gibson's Avatar
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    Pressures are measured with special equipment and test guns. It is expensive. Even with factory measured pressures there is no guarantee that, even if using the exact same components, you will get the same psi in your gun. It may be lower (probably, but it also can be higher. Tested manual loads have conformed to SAAMI test standards (most often) for some time. Those tested loads have +/- factors built into them but since you probably don't have the exact same components and certainly don't have the same test gun maximum loads must be worked up to watching for pressure signs and backing off when those signs appear even if that load is not a "maximum" load. Unfortunately many internet educated reloaders want to jump to a "maximum" load without working up to that load in their firearm. That can be hazardous to the firearm and/or the shooter.

    There is no formula that can accurately predict the correct pressure. QuickLoad is an approximation and can come close but the input data must be correct. Input of not only correct data but also sufficient data is also necessary.

    Understand that measuring chamber pressure is not a direct observation but is indirectly observed by measuring the effect of a secondary occurrence such as how much a lead/copper pellet is crushed, the barrel is stressed or with a transducer. Even then it is only a guess. Different pressure measuring fixture/guns give variations of measured pressure with the same ammunition which is why "reference" ammunition is used to "calibrate" the test barrel/fixture with. Actually the use of reference ammunition simply gives a +/- offset figure to add or subtract from the "measured" psi. That is also why pressures, like velocities, have an extreme spread (+/-) and require a certain number of tested shots to get a meaningful probability, usually 10 shots which is the SAAMI standard.
    Larry Gibson

    “Deficient observation is merely a form of ignorance and responsible for the many morbid notions and foolish ideas prevailing.”
    ― Nikola Tesla

  8. #8
    Up until this time i have used only factory ammo, so ive not had to concern myself as much with preasure. Ammo companys want to avoid law suits so they stay well below max preasures in general. So many of the "numbers" that nevee really concerned me before, now they do. Plus now im actually learning what they mean. Such as with the three tiers for 45 Colt. Original SAA being 14,000, ive never considered to be strong loads. Tier two i felt to be decent, but nothing like the 32,000 for Ruger only loads. That was my PAST line of thinking. But now after learing more, my opinion of the tier two loads have definitely changed. Pushing closer to that 20,000 mark is serious loads. Amazing how your opinions change once you start learning and understanding what all the different numbers actually mean.

  9. #9
    Exactly. I've always shot Rugers but even at that I've tried keeping my loads towards the bottom of the chart. I do have my hunting load which is pretty hot. Its a 300gr soft tip with 23gr of h110. Its a handful. I'm about to load some 325gr cast bullets for hunting. I'll look for a happy medium between my pistol and rifle. Luckily both like the 300gr bullet and load.

    Sent from my XT1650 using Tapatalk

  10. #10
    Boolit Master


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    Pressures are determined by electronic measuring devices. They can be referred to as transducers, elastometers or plastometers. Barrel steels actually stretch when a round is fired and then returns to it original size. The amount the material stretches is proportional to the pressure. This stretch is within what is called the elastic range of the steel. As long as the pressure stays within the elastic limit no damage is done to the steel.
    Think of a coat hanger , it can be bent slightly and when released will go back to its original shape. Exceed the elastic limit and the wire will be permanently bent.
    Again the amount of stretching is related to the pressure. With time and the stretch the pressure curve can be determined or measured. The pressure curve is very important in gas operated semi-automatics. The point the gas is bleed into the gas system makes the difference if the action cycles. Thus you have carbine,mid length and rifle length in the ar-15 platform. Most quoted pressures are peak pressures. Pressures are mostly linear meaning that if a high and low pressure and load are given then a middle load will yield a pressure in the middle of the pressure range.
    Many factors can affect the pressure,ambient temp.,burn rate of powder, strength of ignition,crimp of bullet, bullet resistancestors in bore or if bullet is touching the rifling.
    So there is no way the average person can measure pressure.

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Abbreviations used in Reloading

BP Bronze Point IMR Improved Military Rifle PTD Pointed
BR Bench Rest M Magnum RN Round Nose
BT Boat Tail PL Power-Lokt SP Soft Point
C Compressed Charge PR Primer SPCL Soft Point "Core-Lokt"
HP Hollow Point PSPCL Pointed Soft Point "Core Lokt" C.O.L. Cartridge Overall Length
PSP Pointed Soft Point Spz Spitzer Point SBT Spitzer Boat Tail
LRN Lead Round Nose LWC Lead Wad Cutter LSWC Lead Semi Wad Cutter
GC Gas Check