How close is my fired rifle bottleneck case to actual chamber dimensions?

Discussion in 'Handloading and Reloading' started by IMtheNRA, Feb 19, 2018.

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  1. IMtheNRA

    IMtheNRA Member

    Dec 24, 2002
    My understanding is that brass stretches to fill the chamber, but then it collapses as pressure drops. None of what I read mentioned how much it actually collapses in percentage terms or thousandths. I'm wondering how close is the case (especially the shoulder position) to actual chamber dimensions.
  2. Blue68f100

    Blue68f100 Member

    May 25, 2011
    Piney Woods of East Texas
    It normally takes 3 firings if your neck sizing to get a very accurate impression of your chamber. As far as terms in %, too many variables to make a call, brass hardness, load, chamber specs..... As far as sizing is concern on a bolt rifle you only need to maintain the 1st firing provided it showed no indications of over pressure. Over pressure conditions stretch the brass more. It's normally recommended that you only need to push the shoulder back 0.001" on a bolt gun. For a simi-auto you need to go back to SAAMI specs for reliable feeding.

    If you have a way to measure the brass as SAAMI does. Measure a new un-fired one, then fire it and compare. Then you will have the results of that 1 test.
    Patocazador likes this.
  3. Walkalong

    Walkalong Moderator Staff Member

    Nov 20, 2006
    Hammer it hard (Full power load) and it is close with new soft brass. I have never tried to measure the difference. I tend to move the shoulder back and average.002 from there for bolt guns and average .003 (Or size to fit a case gauge where the average case falls in the middle) for autos. Works well enough. It's easy to get it too tight and easy to get more head clearance than needed. Sometimes it measures "OK", but you will still get a little "bolt rub" going on, which I don't want.
  4. Sofie007

    Sofie007 Member

    Jul 4, 2017
    Depends on the purpose of your question. If it's purely academic, you can theorize all you want and knowing that the cartridge will exactly match your chamber when it's fired but there is some recovery as it cools of course which makes it able to be extracted. Be aware that through series of firing of that one cartridge, the recovery will be less, as fatigue of the metal increases.
    If your question is for practical purposes, you can slug your chamber and then measure cartridge after it's fired to get the exact difference.
    Bottom line: There is no easy answers or even a rule of thumb. For instance, the thickness, chemistry, and physical properties of base metal in the case, the size of your chamber at a specific temperature, the force witnessed on the case during combustion, on and on.
    Last edited: Feb 20, 2018
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  5. Ireload2

    Ireload2 Member

    Mar 19, 2009
    The 7.65 Mauser cartridge used in the 1891 and 98/09 Argentine Mauser is not standardized by SAAMI.
    In order to understand the dimensions for the chamber length I formed cases that produced a very slight drag when the stripped bolt was closed on them in the chambers of 4 rifles. All 4 chambers were within .002 of the same length using a .375 datum circle. These formed cases were considered an accurate transfer of the chamber dimension from the case head to the shoulder.

    Then I checked the length of new Norma brass and loaded factory ammo. The Norma cases and ammo varied between .003 and .004 shorter than the 4 Mauser chambers.
    I also checked a lot of PPU new brass and factory ammo.
    The PPU ammo and brass measured .001 to .002 shorter than the chamber dimension. This is an excellent fit condition. The PPU cases also were more consistent in length than the Norma cases.

    The European CIP standards organization does have standard drawings for this round. However the CIP dimensioning and tolerancing scheme has different conventions. I used an EXCEL spreadsheet to convert the CIP values to the US SAAMI standard format. The result was my measured chamber lengths matched the CIP numbers.

    A reloader can use a tool like the caliper mounted Hornady case comparator to set his FL die to produce a case that fits his chamber length. So you have significant control over that fit if you put out the effort.

    What a reloader cannot adjust easily are the diameters of a FL sized case at the shoulder and pressure ring near the case head. Those sized case dimensions are about .004 smaller than the same dimensions of the chamber. The clearances at the shoulder and pressure ring are dictated by the loading die and the chamber. Both have manufacturing tolerances.

    Some bench rest rifles have smaller than normal chambers to improve accuracy. Some rifles like the #4 Lee-Enfields have a chamber that is both larger in diameter and much longer than most chambers. Without extra care these rifles give terrible case life due to excessive stretching of the cases
    Last edited: Feb 21, 2018
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  6. Varminterror

    Varminterror Member

    Jul 17, 2016
    If a guy studies the results from the AMU’s work over the years, we’d never shoot anything but new, resized brass in competition.

    In practice, almost all matches are won and all records are set with reloads.

    I say that really only to instead say this - don’t overthink it.

    New brass tends to spring back more than brass with multiple firings. If I necksize only, I tend to see my brass “grow” for the first few firings, 3 or 4. Annealing can wrinkle this - better fireforming, but also more resiliency...

    If you really want to know the difference, get some cerrosafe and your micrometer.
    Walkalong likes this.
  7. spitballer

    spitballer Member

    Dec 20, 2013
    Central FL
    Not to sidestep your question but frankly I'm surprised to learn that others go to the trouble of setting back the shoulder of a fire formed case for a bolt rifle - as a practical matter I've never had to do anything but neck size and trim when they get long. Not doubting, just surprised.
  8. Walkalong

    Walkalong Moderator Staff Member

    Nov 20, 2006
    You're surprised some people like to FL size?
  9. Slamfire

    Slamfire Member

    Dec 29, 2006
    I have shot with and talked with AMU shooters, great guys.


    Something they pointed out, their ammunition has to work in everyone's rifle. And it is beyond my comprehension, the number of rounds they shoot. The Bullseye shooters told me they were shooting between 5000-7500 rounds per month of 45 ACP. They also shot 9mm and 22 lr, probably the same amount. I did not ask the rifle shooters the level of lead pollution they were creating , but it could be similar.

    Given the quantity of ammunition, it is far less costly to pay a commercial firm to make them new ammunition, than dedicate a small arm of technicians reloading ammunition.

    I think that case clearance after firing will depend on case composition, case hardness (which varies up and down the sidewalls and through thickness), therefore young's modulus, and pressure of combustion.

    This sort of helps what I am talking about:


    This discussion over steel versus brass case material might give some insight:

    John Bercovitz ([email protected])
    I see that that article was sort of long. Let me preface it by giving you a bare bones synopsis. A brass cartridge case expands past its yield point during firing, at least for the first firing when the case doesn't fit the chamber so well. That is to say, it stretches plastically to some new size and then springs back from that new size to its new relaxed size.
    If that springback is greater than the springback of the steel chamber, then the case will extract easily. If not, the case will want to be larger than the sprung back chamber and so it will be a "press fit" in the chamber and will be difficult to extract. This extraction difficulty appears approximately when chamber pressures exceed 70,000 psi.
    Here's the article:
    A friend asked why steel cases aren't more common since they would allow higher chamber pressures. I thought that as long as I had written something up for him, I might as well post it here:
    Material Properties
    CDA 260 cartridge brass: barrel steels:
    Young's modulus = 16*10^6 psi Young's modulus = 29*10^6 psi
    Yield stress = 63,000 psi min. Yield stress: usually > 100,000 psi
    I was going to get back to you and explain further why brass is a better cartridge case material than steel or aluminum. Sorry I took so long. I left you with the nebulous comment that brass was "stretchier" and would spring back more so it was easier to extract from the chamber after firing. Now I'll attempt to show why this is true given the basic material properties listed above.
    A synopsis would be that the propellant pressure expands the diameter of the thin wall of the cartridge case until it contacts the interior wall of the chamber and thereafter it expands the case and the chamber together. The expansion of the cartridge case, however, is not elastic.
    The case is enough smaller in diameter than the chamber that it has to _yield_ to expand to chamber diameter. After the pressure is relieved by the departure of the bullet, both the chamber and the cartridge case contract elastically. It is highly desirable that the cartridge case contract more than the chamber so that the case may be extracted with a minimum of effort.
    A quick review of the Young's modulus: this is sort of the "spring constant" of a material; it is the inverse of how much a unit chunk of material stretches under a unit load. Its units are stress / strain = psi/(inch/inch).
    Here's a basic example of its use: If you have a 2 inch by 2 inch square bar of steel which is 10 inches long and you put a 10,000 pound load on it, how much does it stretch?
    First of all, the stress on the steel is 10,000/(2*2) = 2500 psi.
    The strain per inch will be 2500 psi/29*10^6 = 0.000086 inches/inch.
    So the stretch of a 10 inch long bar under this load will be 10 * 0.000086 = 0.00086 inches or a little less than 1/1000 inch.
    Yield stress (aka yield strength) is the load per unit area at which a material starts to yield or take a permanent set (git bint). It's not an exact number because materials often start to yield slightly and then go gradually into full-scale yield. But the transition is fast enough to give us a useful number.
    So how far can you stretch CDA 260 cartridge brass before it takes a
    permanent set? That would be yield stress divided by Young's modulus:
    63,000 psi/16*10^6 psi/(inch/inch) = .004 inches/inch.
    How far can you stretch cheap steel? Try A36 structural steel:
    36,000 psi/29*10^6 psi/(inch/inch) = .001 inches/inch.
    How about good steel of modest cost such as C1118?
    77,000 psi/29*10^6 psi/(inch/inch) = .003 inches/inch.
    (Note that C1118 doesn't have anywhere near the formability of CDA 260. Brass cases are made by the cheap forming process called "drawing" while C1118 is a machinable steel, suitable for the more expensive machining processes such as turning and milling.)
    What about something that's expensive such as CDA 172 beryllium copper?
    175,000 psi/19*10^6 psi/(inch/inch) = .009 inches/inch.
    (This isn't serious because CDA 172 is pretty brittle when it's _this_ hard. Not to mention that beryllium is poisonous even in an alloyed state.)
    Note: I was corrected on this. It seems that very low alloys of beryllium are _not_ considered poisonous (except here at this Lab. 8-))
    Titanium Ti-6AL-4V
    150,000 psi/16.5*10^6 psi/(inch/inch) = .009 inches/inch
    (This is an excellent material though expensive and hard to work with.)
    Really expensive aluminum, 7075-T6
    73,000 psi/10.4*10^6 psi/(inch/inch) = .007 inches/inch
    Cheap aluminum, 3003 H18
    29,000 psi/10*10^6 psi/(inch/inch) = .003 inches/inch
    (Aluminum isn't a really good material because it isn't strong and cheap at the same time, it hasn't much fatigue strength, and it won't go over its yield stress very often without breaking. So you can't reload it. It makes a "one-shot" case at best. Also, 7075 is a machinable rather than a formable aluminum, primarily.)
    Magnesium, AZ80A-T5
    50,000/6.5*10^6 = .0077
    (Impact strength and ductility are low. Corrodes easily.)
    +Here's the important part: Even if you stretch something until it +yields, it still springs back some distance. In fact, the springback +amount is the same as if you had just barely taken the thing up to its +yield stress. This is because when you stretch it, you establish a new length for it, and since you are holding it at the yield stress (at least until you release the load) it will spring back the distance associated with that yield stress. So the figures given above such as .004 inches/inch are the figures that tell us how much a case springs back after firing.
    Changing subjects for a moment: How much does the steel chamber expand and contract during a firing? Naturally this amount is partially determined by the chamber wall's thickness. The outside diameter of a rifle chamber is about 2 1/2 times the maximum inside diameter, typically. The inside diameter is around .48 inches at its largest. Actual chamber pressures of high pressure rounds will run 60,000 psi or even 70,000 psi range if you're not careful.
    One of the best reference books on the subject is "Formulas for Stress and Strain" by Roark and Young, published by MacGraw-Hill. Everyone just calls it "Roark's". In the 5th edition, example numbers 1a & 1b, page 504, I find the following:
    For an uncapped vessel:
    Delta b = (q*b/E)*{[(a^2+b^2)/(a^2-b^2)] + Nu}
    For a capped vessel:
    Delta b = (q*b/E)*{[a^2(1+Nu)+b^2(1-2Nu)]/(a^2-b^2)}
    a = the external radius of the vessel = 0.6 inch
    b = the internal radius of the vessel = .24 inch
    q = internal pressure of fluid in vessel = 70,000 psi
    E = Young's modulus = 29 * 10^6 psi for barrel steel
    Nu = Poisson's ratio = 0.3 for steel (and most other materials)
    A rifle's chamber is capped at one end and open at the other but really it's not too open at the other end because the case is usually bottle- necked. You'd have to go back to basics instead of using cookbook formulae if you wanted the exact picture, but if we compute the results of both formulas, the truth must lie between them but closer to the capped vessel.
    For an uncapped vessel:
    D b = (70000*.24/29*10^6)*{[(.6^2+.24^2)/(.6^2-.24^2)] + .3} = .00097
    For a capped vessel:
    D b = (70000*.24/29*10^6)*{[.6^2(1.3)+.24^2(.4)]/(.6^2-.24^2)} = .00094
    There's not a whole heck of a lot of difference between the two results so let's just say that the chamber's expansion is .001 inch radial or .002 inch diametral.
    The cartridge case's outside diameter is equal to about .48 inch after the cartridge has been fired. So its springback, if made from CDA 260, is
    .004 inches/inch (from above) * .48 inch = .002 inches diametral
    which of course is just the amount the chamber contracted so we've just barely got an extractable case when chamber pressures hit 70,000 psi in this barrel. This is why the ease with which a case can be extracted from a chamber is such a good clue as to when you are reaching maximum allowable pressures. By the same token, you can see that if a chamber's walls are particularly thin, it will be hard to extract cases (regardless of whether or not these thin chamber walls are within their stress limits).
    A really good illustration of this can be found when comparing the S&W model 19 to the S&W model 27. Both guns are 357 magnum caliber and both can take full-pressure loads without bursting. The model 27 has thick chamber walls and the model 19 has thin chamber walls. Cartridge cases which contained full-pressure loads are easily extracted from a model 27 but they have to be pounded out of a model 19. So manufacturers don't manufacture full-pressure loads for the 357 magnums anymore. 8-(
    We can see from the above calculations that a steel case wouldn't be a good idea for a gun operating at 70,000 psi with the given 2.5:1 OD/ID chamber wall ratio if reasonable extraction force is a criterion. Lower pressures and/or thicker chamber walls could allow the use of steel cases.
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