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Shotgun Headspacing and Foreign Ammo

Discussion in 'Ammunition' started by CaddmannQ, Mar 28, 2017.

  1. CaddmannQ

    CaddmannQ 12g Supporter

    I found something interesting on Shotgun World, and thought it was particularly worth reposting.

    I know all common shotguns headspace on the rim, like most old-fashioned cartridges do, but I didn't consider the difference between American and foreign guns and American and foreign ammunition!

    And I should have, because as an engineer I've worked on lots of Metric-American projects before, for both construction and manufacturing.

    Generally speaking, different industries use different "soft" metric conversions & foreigners use "soft" American conversions. This means you don't just put two numbers into a calculator and come up with the size.

    Those conversions get "doctored" to allow for a number of manufacturing realities (which are too involved to explain right here.)

    So remember this when you stick a Rio in that Mossberg.
    . .

    ( the following is reposted from shotgunworld.com)

    This is something that (the late) Ralph Walker had to say about shotshell headspace.....

    "A few years ago I was on an assignment to a manufacturer in Brazil who was exporting a shotgun to the United States. My job was to solve some problems and to arrive at a set of specifications on the chamber. A sample gun was built at the research and development department of the factory and I thoroughly tested it on the company range. The sample gun performed flawlessly with excellent shooting results. As all testing was done with shotshells manufactured in Brazil, the final step was to bring the gun back to the States for the same tests with American shotshells.
    Pattern performance was good, but the gun offered far more recoil than I had experienced in Brazil. I asked for the shotshell data from the manufacturer and went over this thoroughly comparing it to known pressures and velocities of American shotshells. There was little difference that would account for the difference in recoil. I then asked for specifications of the chamber which was, of course, in metric. There was the culprit!
    The Brazilian shells were manufactured with a thicker rim and the chamber recess for that rim was cut deeper to match the shell. By SAAMI specifications, the recess was so deep that it did not meet the maximum allowed tolerance. To make a long story short, another R&D gun was manufactured to American specifications and shipped to me for testing. With the same shell that was used in the first test, the excess recoil disappeared. The production model of the gun then was made with a new set of specifications to match the American shell and this part of the problem was solved.
    Why should the depth of the rim recess make such a difference? This requires an explanation that is not commonly considered when a shotgun is repaired by a gunsmith.
    Said one way, headspace is the distance between the rear of a fully chambered shotshell and the face of the fully closed breech or breech bolt. Another definition is the distance from the face of the fully closed breech and the forward section of the recess cut in the chamber. It all depends on how you are measuring this space. Personally I prefer the latter definition, as it is a bit easier to work with.
    Headspace gauges manufactured in the United States consist of two solid metal plug type gauges. The only difference is the thickness of the rim of the gauge. One is marked minimum, the other maximum and the gun's gauge is designated.
    The minimum gauge simply means that when the gauge is inserted into the chamber and the gun is fully closed, it should close without difficulty. If it does not close, the recess cut for the shotshell rim is not deep enough to accept a standard American shotshell. With this gauge removed and the maximum gauge inserted into the chamber, an attempt is made to fully close the breech. It should not close. If it does close, the rim recess is cut too deeply and the chamber has excessive headspace.
    Excess headspace in a shotgun chamber is not as critical as in a rifle chamber, but it still should not be taken lightly. There is a danger point in this situation; for excess headspace means that the rim of the shotshell is not fully supported by the surrounding chamber. In a rifle, the cartridge is quite thick at this point and will forgive a multitude of sins. The shotshell rim usually is made of folded metal and is considerably weaker. In extreme cases, the rim will split or rupture and release gas from the shell. I have seen this happen and it is not a pleasant feeling to have gas spewing that close to your face. Under these conditions a fired shotshell may have a split rim or the rear of the shell may be blown off to leave nothing to contain the gas pressure. In a shotgun chamber, such pressure is in the neighborhood of 10,000 pounds per square inch (psi).
    Even if this pressure is not reached, the rear of the shotshell can be bulged, making the case useless for reloading, but more important, the gun will not properly extract or eject the fired case. On some pumps and semi-autos, the brass rear of the shotshell is torn loose and the forward section of the shell is left in the chamber. This can create considerable safety problems, but at least another shell will not chamber and thus you have a feed condition that could be mistaken as to source of probable cause. In short, headspace can affect three things on a shotgun: failure to correctly extract, eject or feed the next shell. Every shotgun chamber should be checked with headspace gauges before any conclusions as to root cause of a problem is decided by the gunsmith; yet this is often overlooked or ignored in shotgun repair.
    It is good practice to check every gun not manufactured in the United States with the gauges, as foreign shotshells are not made to the specifications of the American shotshell. Incidentally, this can be considered a point of national pride, ad no other country produces a shotshell even close to the quality and precision of American shotshells. I have fired a lot of foreign-made shells and can testify as to the truth of the foregoing. So,if the gun is made overseas, hold it in suspect until the chamber is checked with the minimum and maximum headspace gauges.
    What actually happens when a shotshell is fired in a chamber in which the rim recess is cut too deep? The striker or firing pin drives the shell forward until it is halted by the front shoulder of the cut recess. The primer is ignited and in turn ignites the powder charge. The resulting gases expand the walls of the shotshell until they obturate or seal to the inner ways of the chamber under normal conditions.
    When excess headspace is present, before obturation can occur, the gas pushes the shotshell to the rear and slams it against the breech. This slam force creates a rearward thrust on the complete gun. Added to the force of the shotshell in it's rearward thrust of the gun, which we call recoil, you have a combined thrust and excessive recoil is felt on the shoulder of the shooter.
    Degree of extra recoil depends on the depth of the recess cut in the chamber rear and the rim of the shotshell. The more space created, the more the slam force. If a barrel is corrected to give zero space, my own experience suggests reduction of between ten and twenty percent. There is a definite difference before and after headspace is corrected
    MikeD likes this.
  2. John A.

    John A. I'm "THAT" guy Staff Member Global Moderator Moderator

    Maybe that's why my Turkish O/U kicks like a mule?
  3. CaddmannQ

    CaddmannQ 12g Supporter

    I am not convinced that the diff in perceived recoil was 100% the result of headspace only. Equal max pressures do not mean equal recoil energy. I think the US ammo had better powder. Not hotter, but releasing more total energy.

    At least that's my theory.

    IMO the primary issue with loose headspace will be Fail To Fire on the first strike, because the shell moved too much when struck.

    Actually the foreign ammo works OK in my US made guns so far.
  4. Ghmann

    Ghmann .270 WIN

    I have shot Fiocchi, Rio, Aguila, & S&B in my 590 with no problems.
  5. CaddmannQ

    CaddmannQ 12g Supporter

    Generally they will all work in a USA made gun, unless you get a tight new gun with a real tight headspace and it doesn't want to close on them. Also you'll probably notice that the extractor leaves heavier marks in those foreign shells, as it hits them a little harder.

    I don't know if this could lead to premature extractor damage, but probably not, as my shotgun extractors seem pretty stout.

    The problem comes with misfires, from using US shells which will fit a little too loose in foreign made guns, with the metric-spec chamber.

    Anyhow I recently picked up about 400 assorted free 12 gauge shells at the gun range and reloaded them.

    I rejected quite a number of shells that would have normally passed because they were foreign-made shells with heavy extractor marks in the rim. Some of them had aluminum bases and I often found those with extremely heavy ejector marks and large burrs. I tossed most of the aluminum ones, but there were about a dozen really clean ones which I did reload.

    Most of the usable shells were steel plated with brass, and show far less ejector marking. The US brands were basically all cleaner there.

    Bear in mind that this was 6 different brands of shells, probably shot with 8 to 12 different guns.

    My shotgun extractors don't seem to make any big marks, but we'll see what happens when I shoot those aluminum base reloads.
  6. CaddmannQ

    CaddmannQ 12g Supporter

    Does it do the same with foreign-made shells?

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