IMR and Hodgdon 357 reloading data

I suppose that the original .357 Magnum REVOLVER barrel length of 8 3/8", when added to a cylinder length of slightly more than cartridge length (1.610") looks a lot like 10" in round numbers. So, that may be where the 10" test barrel length originated.

But, even with a test barrel that matches the barrel + cylinder length of a revolver, the test barrel almost always produces higher velocities. Some folks thought that the main difference was the gap between the barrel and cylinder face venting powder gases. Some special "vented" test barrels were produced to try to address that effect. Lyman's manual used to publish some .357 Magnum load velocities from a 4" vented test barrel. But I don't really know if the whole barrel was only 4" long, or if there was 4" beyond the vent.

One reason that at test barrel produces higher velocities than most retail fire arms is that the test barrels are intentionally produced with dimensions that will maximize peak pressure. That means that pressures measured in test barrels will not be exceeded by pressures in guns sold at retail so long as those retail guns are within SAAMI dimensional specs. It also means that the lower pressures in retail guns will prodeuce lower velocities with the loads tested. (However, pressures in revolvers may not be so easy to bound in that manner.)

Some manuals try to give handloaders a more realistic expectation of what velocities the listed loads will produce by shooting them in an example retail firearm after the max pressures have been checked in a pressure barrel. That is helpful, but still not very helpful. The problem is that the velocities of the exact same .357 Magnum loads in various revolvers of the same barrel length can vary by as much as 200 fps, even between two revolvers from the same manufacturer. For a good demonstration of that variability, look at the table on page 771 of Speer Manual #14.

It should also be pointed out that the ACTUAL peak pressures in revolvers can be quite different than what the same loads produce in a test barrel with the same internal dimensions. Speer demonstrated that by making a pressure testing device to replace the cylinder in a standard revolver. The peak pressure in a revolver can actually be higher than in the test barrel. That appears to be due to the forcing cone that is in the revovler, but not the test barrel, even the "vented" test barrel. Sometimes, a bullet will slightly "rivet" in the forcing cone, expanding to more than the cylinder throat diameter where it is not supported by the cone, then getting squeezed down to bore diameter. That is where the pressure may peak in a way that the test barrel does not duplicate.

So, in reality, ammo that meets SAAMI peak pressure limits in a test barrel may exceed that pressure in real revolvers in some situations. From years of experience in measuring pressures in test barrels and designing revolvers to hold-up when the same loads are shot, we have an industry that produces guns and ammo that can be shot without fear of the guns coming apart, even if the acutal pressures in the revolvers are not really known with much accuracy.

But, trying to match test barrel velocities by increasing handload charges above published max loads is NOT safe, because that can produce peak pressures that are above what the test barrels produced, and above what our guns were designed for.

SL1

Last edited by SL1; Yesterday at 01:09 PM.

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