Measuring the "WEB" area of the case for pressure signs?

Discussion in 'Reloading' started by KQguy, Dec 2, 2008.

  1. KQguy

    KQguy Well-Known Member

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    I have heard of reloaders measuring the "web",checking for pressure signs on cases.I am interested in using this approach,but I don't know exactly where this area of the case is,and how the procedure works.I have heard it is the safest,and most accurate way of determining pressure signs.I was hoping there are some people here who knows how this method works,and could chime in with some good info. on it.
     
  2. MagnumManiac

    MagnumManiac Well-Known Member

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    KQ,
    To measure case head expansion, you need a micrometer that reads to .0001" (1 ten-thousands of an inch). A normal .001" micrometer isn't accurate enough for this type of measuring.

    You will also need to 'scarifice' around 5 cases per batch if they are rimless or belted. Rebated rims do not need this done.

    To accomplish accurate readings, you need to index (draw a line across) the bottom of the cases with a felt tipped marker, and then at opposite ends as indicated by the indexing, you need to file off about .003" of the rim, very carefully so as not to touch the case body, this is so the jaws of the micrometer fit at the very rear of the case body just forward of the extraction cannelure, this is where the measurement is taken. If you have 'blade' type micrometers, you won't need to do this.

    The procedure is to take your reading as described above, a few measurements at the same place is the most accurate, you can take a measurement with the case one way and then rotate it 180 degrees and measure again. Take note of your measurement. This measurement is to be taken BEFORE firing the case.
    After firing the case, take another measurement at EXACTLY the SAME place on the case and record that measurement.
    The difference between the 2 is the case head expansion; .5312" before firing and .5315", or .0003" larger, after firing.
    I do this with 5 cases from the same batch for each load increment.
    You should discard these cases after 3 firings, because work hardening can set in and give you erroneous readings. Always start with once fired (starting loads) brass, because the first deflection is already present.

    It is a general rule that .0003"-.0005" is around 50,000cup, but I prefer to work off .00025" as maximum expansion allowed.
    Once this is reached, I then reduce my loads so that no expansion, or only .0001" is detectable.

    It must be noted, however, if ANY one case, from the same batch, shows expansion above .0005", the load is TOO HOT and you will need to reduce the charge back to the load that showed no expansion or only .0001" for safety. A change in ambient temp may cause the previous HOT load to go way over and ruin your day and firearm!
    Hope this helps, if not, I suggest you buy the latest Speer Manual #13, which describes the same procedure.
    Cheers.
    MagnumManiac.
    gun)
     

  3. KQguy

    KQguy Well-Known Member

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    Thanks for the info.,do you feel this is the best method for checking pressure signs?
     
  4. MagnumManiac

    MagnumManiac Well-Known Member

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    KQ,
    Unless you own a pressure sensor as I do, I believe this is the only SAFE way of determining SAFE PRESSURES for your brass, it is not the rifle that you have to be concerned with, it is the safety of your brass cases that is important.
    There are too many variables and inconsistencies when handloading. It is foolish to believe that primer appearance is an infallable measure to what is a safe load in any cartridge.
    The only infallable sign of an over max pressure load is the appearance of an ejector/plunger mark on the head of your cases, it actually doesn't matter whether you have an overload or not, if this is present on even ONE case at that loading, it is too hot for that brass, the next step is a ruptured case, and a blown up rifle.
    Until I got my pressure sensor, case head expansion was the only way I measured my handloads for the OPTIMUM SAFE LOAD available in that brass, it didn't matter whether the loads were in fact at maximum SAAMI pressure or not, they were compltetely safe in my brass and rifles, and in fact gave better performance, in most instances, than my buddies who loaded until they felt 'sticky' extraction, and backed off 2 gr of powder and called that their safe max.
    Cheers.
    MagnumManiac.
    gun)
     
  5. KQguy

    KQguy Well-Known Member

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    Thanks for all the info.,I have never felt very confident checking for pressure signs by looking at primers,and extractor marks.
     
  6. Mikecr

    Mikecr Well-Known Member

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    You'll get every pressure sign in the world before getting in trouble. But watching web expansion can help you save brass from a life shortening standpoint.

    On upward load changes I check & back off with significant step changes measured in front of the extraction groove. I just use a caliper for quick checks at the range. But this is with well formed brass that has FULLY SETTLED.
    Some cartridges, some brass, and some accurate loads, produce changes different than others. So you have to take this into account and just get to know your brass..
    For example, growth from a 223 is considerably different than same measured growth from a WSSM, -given the same pressures.
    And after FL sizing, you can't go by any kind of before/after comparison.

    It just comes down to a step change from what you normally measure, from fully fire-formed brass, as freshly pulled from a smoking chamber.
     
  7. Clark

    Clark Well-Known Member

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    I have overloaded allot of cases to see what happens.

    A) The 10mm is the weakest case head. It has a large Boxer primer pocket and deep extractor groove, making a thin spot in the web that sees the radial expansion of gas pressure and axial compression of the case pushing against the the breech face.

    B) The 1889 7.65x53m Mauser case head fitted with Large rifle Boxer primer pocket is stronger and good for ~ 62,000 psi for long brass life. There are many cartridges that use this; 22-250, 243W, 6mmRem, 250 Sav, 257 Roberts, 25-06, 260 Rem, 270, 7mm-08, 7x57mm, 280 Rem, 308W, 30-06, 8x57mm, 338 Federal, 338-06, and 35 Whelen.

    C) The 6mmBR case head has Mauser dimensions, but uses a small rifle Boxer primer pocket. This makes the web so strong that the primer is the weakest link. A CCI450 magnum primer and a custom bushed firing pin allow the usable pressure to be very high.

    What does it all mean?
    In some cases, the web is the weak link, and loads must be adjusted for long brass life by monitoring the primer pocket growth to insure there are no loose primers. Other webs are built better.
     
  8. milanuk

    milanuk Well-Known Member

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  9. Clark

    Clark Well-Known Member

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    Over the years at Accurate Reloading forum I have endured hundreds for flame wars between Denton Bramwell "Denton" measure pressure advocate and Glenn Rice "Hot Core" measure brass advocate.

    I think they are both electrical engineers, as am I.

    After that much arguing, it is a stylized debate.

    I have my own flow chart:
    1) If I were to sell ammo, I would measure pressure* and develop
    loads to SAAMI registered pressures.
    2) If I am loading for a gun that is weaker than the brass, like a revolver with .040" thick chamber walls and a rimmed case, just don't exceed max loads in load books, where pressure was measured.
    3) If I am loading for a gun stronger than the brass, load up to the threshold of short brass life, and reduce by at least a safety margin to find the max load for my gun.

    * With CEA-O6-250UW-350 strain gauges and AD8554 op amps in an instrumentation amplifier configuration, I can get great accuracy in measuring the strain on barrel steel locally.
    Instrumentation amplifier - Wikipedia, the free encyclopedia
    One problem I have is the precision of where the gauge is epoxied onto the barrel.
    Another problem I have is that the barrel is not a uniform thickness thin wall long tube. It is an open ended tapered tube. That gets me into Roark's book on stress vs stain on open ended tubes:
    Amazon.com: Roark's Formulas for Stress and Strain: Warren Young, Richard Budynas: Books
    I think Denton buys amplifiers, and calls mine "home brew", but mine are more accurate. The inaccuracy is in the epoxy, position, thickness, and formula for tapered open ended barrels. Then there is the error of value used for Young's Modulus for steel in the barrel.
    I have a friend who has by passed this, and gone to pressure receiver and transducers.
    That still leaves errors for the handloader, in that it misses the individuality of the gun and the error of the pressure registered with SAAMI.
     
  10. milanuk

    milanuk Well-Known Member

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    Clark,

    If a person didn't have the instrumentation and chose to go with measuring case head expansion... what would you consider a reasonable thresh hold to say 'No more'?

    The reason I ask is this: a few years ago I had called Sierra tech support about one of their bullets (the new at the time 90gr .224 SMK), and took the opportunity to ask them why their published 'max' loads were so far below what seemed to be in relatively common use. To put it bluntly, why their loads seemed so pussified.

    I expected the usual hemming and hawing about corporate lawyers and such, but what I got surprised me even more. The gentleman (don't recall the name) said they measured CHE with blade micrometers and when the case head expanded some specific amount (0.0010" rings a bell, but unfortunately I didn't write it down), that was it. Max load, period, based off expansion of the case head as measured in the extractor groove (to my understanding).

    To say I was somewhat shocked that a major bullet manufacturer who publishes such an extensive loading manual was relying on such a low-tech and indirect method of pressure measurement would be an understatement. I are not an engineer (I just get to deal with the consequences of their work :D) but that seems a little weird.

    Monte
     
  11. Clark

    Clark Well-Known Member

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    The Sierra rifle book is good, it stays the same for decades. I work up loads and come back down from problems and wind up close to their loads often enough.

    They are nice to talk to or email.
    I wrote them about the error in their pistol book that said that CZ52s were stronger than Tokarevs. I told that that it measures and calculates the other way. They wrote back that they just wrote what they read. That was refreshing to get the truth instead of a politically correct answer.

    The Mauser case head used on a 308 is SAAMI rate for 62kpsi and is only good to 62kpsi.
    The .222 case head used on a 223 is SAAMI rated at 52kpsi and is good to 72kpsi.

    This is a problem for the 22-250.
    It is manufactured with a slow twist barrel, only good for light bullets, but the .223 is practically as good with light bullets if both cartridges are loaded to the threshold of short brass life and backed off a safety margin.

    The only niche for the 22-250 with advanced handloaders would be with a custom fast twist barrel.

    But I will assume you have a stock rifle and only stock .223s have the twist rate to launch a 90 gr bullet with stability.

    I would assume that Sierra was loading to 52kpsi, and not 72kpsi.
     
  12. Mikecr

    Mikecr Well-Known Member

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    There is never going to be a standard, rule of thumb, or method that will apply appropriately across the board here.
    Too many local factors..

    It would be like predicting 'accuracy' from a barrel without having established a mountain of parameters and rigid definitions. Something that hasn't happened to date...

    So if my load and dies produce a 5 reload case life to get best accuracy, and I can live with it, there is no problem. If I put alot of work into my brass, and need 20+ reloads from it, I'll watch growth very closely with my barrel, cartridge, chamber, brass, resultant ES and grouping, etc.
    You cannot predict results here through procedure.

    Just get to know your brass.
    Look at each round pulled and learn what you can on the bench.
    With this, you'll see that .0005" growth from a 223rem means something different than .0005" growth from a 6.5wssm.