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Old February 6, 2013, 07:03 PM   #1
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Bearing Surface variations question

Reading in my new Speer #14. It says;
Bearing surface is a major player. [in pressure.]
I'm not looking for specific information for a specific load, I'm just wondering how much of a "major player" bearing surface is, in terms of starting load calculation?

For example; I am trying to figure up a starting load for a generic pistol bullet, for which there is no load data. The nearest load data I find is for a Speer pistol bullet of the same weight and jacket material, but the bearing surface on the Speer bullet is is significantly shorter than on the generic for my load. Is there anything more specific about this that anyone can tell me, other than to reduce my starting load by ten percent.
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Old February 6, 2013, 08:09 PM   #2
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It is really difficult to PREDICT the differences between one bullet and another of the same construction type when it comes to peak pressure.

There are some experiments with pressure measuring apparatus that show (for rifle cartridges) that max loads (about 60,000 psi) with one bullet can produce pressures on the order of 150,000 psi with what appears to be a similar bullet. Besides bearing surface, there is also the thickness and hardness of the jacket material and the hardness of the core material. Diameter of the bullet with respect to the throat and bore can also make a significant difference in some examples.

In revolvers, it can get even more difficult because of the "rivet" effect when the bullet enters the forcing cone. There it MAY expand radially from pressure on the base and sudden resistance toward the nose, increasing the force needed to force it into the bore/rifling and increasing peak pressure (which occurs at about the same time). A soft bullet that gives less resistance than a hard bullet with mild loads can instead give more resistance under heavy loads. That is why Hornady makes two versions of some bullets in the same caliber and weight. A soft one that is intended for the .45 Colt can cause excessive pressures if loaded to the same data that was developed for a hard one with the .45 Casull or .460 S&W. On the other hand, the hard ones can produce higher (but not usually catastrophic) pressures in the .45 Colt, and of course, they won't expand properly at .45 Colt velocities.

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Old February 6, 2013, 09:04 PM   #3
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Checking all online data, using that bullet weight, may produce the lowest starting load for a powder/bullet combinitation. Steve's pages is good for this. IMO. I would never use the maximum loads listed, as they may have come from old Speer loading manunals that will produce high pressure. But safe if working up from starting.
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