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Old December 27, 2012, 10:12 AM   #15
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Join Date: November 8, 2007
Posts: 2,001
As far as diameter goes, some ".357 Magnum" barrels may actually measure as small as .355" (Colts in particular), while some "9mm Parabellum" barrels may actually measure as large as .359", and both would still be within SAAMI specs. There is actually a SAAMI limit on the minimum cross-sectional area of the bore + grooves that also comes into play for tight barrels.

With respect to the pressure increase caused by bullets that are over-diameter for the barrel, that is one of the factors that leads to "fast barrels" on some guns. So, it DOES makes some difference in the pressures, evidence of which can often be seen on a chronograph (but not always).

SAMMI pressure limits apply to the charge weights for the tightest barrels, for whatever bullet the manual makers are testing. So, a maximum pressure-tested load will be below the SAAMI peak pressure limit with THAT diameter bullet. Substituting a larger diameter bullet of the same weight at the same seating depth will create higher pressures for sure, but how much higher is not clear. There are also other factors, such as jacket thickness and lead core hardness that can significantly affect pressures, perhaps more than bullet diameter. So, substituting bullets is ALWAYS something to be done carefully.

With regard to pressure signs, there are really none that are very reliable at normal pistol pressures. That is especially true of cartridges like the .45 Colt, which is not supposed to exceed about 20,000 psi, but still true of cartridges like the 9mm, that are not supposed to exceed about 35,000 psi. In an auto-loader, you can tell if your brass is being ejected with more or less force than occurs with factory ammo. But, that is not a very good measure of PEAK pressure, which is what ruptures cases.

When starting to handload, it is best to stick with the load manual recepies as closely as possible until you have mastered the loading processes and learned how to avoid errors there, as well as what to expect from YOUR particular lots of powders and primers. Then, when you start to experiment with things that the manuals don't cover (such as revolver bullets in auto-loaders), you are in a better position to avoid having uncontrolled factors create unexpected and perhaps misunderstood results, and to recognize when things are not going as planned.

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