Detecting Excessive Pressure

Discussion in 'Ammo & Reloading' started by pokute, Oct 23, 2014.

  1. pokute

    pokute Sincere as a $5 funeral

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    Note that I am paraphrasing from Phil B Sharpe's Complete Guide To Handloading.

    There are some cases where one would like to load cartridges to pressures higher than SAAMI standard values. Two justifications for this, both predicated on the presumption that one has a modern gun sufficiently strong to experiment with, are that modern brass is strongly overbuilt, and that some guns are especially strong or can be configured so that their inherent strength is increased.

    The techniques used in revolvers and pistols vary somewhat, so I will discuss each in turn. Note that there is no guarantee that doing anything I describe here is safe. On the contrary, doing any of these things without full understanding of the behavior of the individual gun and cartridge presents an imminent and certain hazard. I PRESENT THIS INFORMATION SOLELY FOR THE PURPOSE OF EDUCATION.

    The first step is to purchase several brands of common commercial cartridge made by well established manufacturers. These cartridges are GUARANTEED to meet SAAMI specifications pertaining to the round as described on the box.

    Carefully measure the diameter of each cartridge along the straight section of the case about 1/16th of an inch above the head or extractor groove, as appropriate. With factory rounds there should be little variation. If you can detect a variation exceeding 0.0005 inches, stop and do not attempt to use the ammo for this test. It is however fine for normal use.

    Fire these cartridges in your gun or guns. At least ten to each gun (50 when you are ready to work up a final load that runs over spec), and retain the spent brass along with full identification of the source cartridge and the gun it was fired in.

    Carefully measure the spent cartridges as before. If a bulge is apparent (based on caliper or micrometer measurement) near the base of the cartridge, measure the largest diameter at the bulge and use that measurement. If there is a visible bulge (commonly seen with factory Glock barrels), STOP. A visible bulge is a sign of incipient failure. Glock owners should consider upgrading to Bar-Sto barrels.

    At this point, the important number is the largest of the spent cartridge measurements. And you now have some brass that has seen as consistent history as possible to experiment with.

    What is the source of the expansion? When a round is fired, it is subjected to a complicated pressure cycle while constrained by the gun. The brass expands to fill the chamber, the chamber is expanded slightly, and the brass springs back, winding up slightly larger than it started. In a revolver, the part of the cartridge supported by the ejection star is relatively unconstrained. In a pistol, the part of the cartridge unsupported by the chamber, usually at the lower rear, is not constrained. Modern cartridge brass is usually very thick at these points to prevent case-head separation or blowout.

    Now you are ready to experiment. Reload the retained once-fired brass as you normally would. Measure it as described above, and shoot some normal loads. If you are satisfied that the expansion is not unusual compared to your previous measurements, you can shoot progressively hotter loads (0.2 grain increments are safe if you are accurately weighing each powder charge - Nothing is safe if you are relying on a thrower). As long as you don't see any statistically significant change in the maximum cartridge base diameter, you are probably safe. If you do see a change, you're gun may already be compromised!

    So, there you are. And you can see why relatively few people attempt this kind of experiment, and why you should be skeptical of loads reputedly based on this kind of thing OR statements like "the primer wasn't flattened much", or "the case wasn't bulged much". Brian Pearce of Handloader magazine does a lot of this sort of thing, and has access to a laboratory capable of backing up his data with real pressure measurements. He sometimes publishes very interesting data on hot loads. I have run some of his loads and had very good results. I recommend careful repeated reading of all of his loading articles. See my post about books, which includes the issue numbers of Handloader with interesting handgun load data:

    https://www.springfieldforum.com/f36/best-gun-books-661

    A surprising result of the increased strength of newer brass is that there is now a MINIMUM pressure required to get a good seal with 45 acp brass in a non-minimum chamber. Failure to achieve the minmum necessary pressure causes decreased accuracy and a telltale lead smear on the outside of fired cases. Target barrels with very tight chambers are necessary when loading down to bullseye competition levels with newer brass.
     
    Last edited: Oct 24, 2014
  2. pokute

    pokute Sincere as a $5 funeral

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    Oops, looks like the edit window closed on that one... I failed to state, at the very end, that very tight chambers are required due to the low pressure of bullseye competition loads, not for the sake of accuracy. A well cut, concentric chamber may be somewhat oversize, as long as the brass case expands to seal the chamber given the load. Bullseye has a peaky burn curve, so it will expand cases with relative lower velocity loads.