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Given a gun capable of handling 35,000psi pressure, the main difference between .38 special and .357 magnum is the amount of space under the bullet. This is conventionally referred to by the cartridge overall length (OAL) specification. The OAL specification for a cartridge with a given bullet will determine the amount of space in there.
Because the .38 Special case is shorter, it has a lower maximum OAL for a given bullet length. The .357 case is .125" longer, so with a given bullet the OAL can be that much greater before the bullet is no longer in the case.
.357, on the other hand, may have a higher practical minimum OAL. If you seat the bullet too deeply, you may not have anything to crimp to. Wadcutters can be deeply seated and the crimp formed over the leading edge, but bullets with ogives and crimp grooves might require an OAL where the bullet's base is far above where it would be for .38 Special.
Because the "powder density" or "% case filling" (as reflected in the OAL specification) is different from .38 Special to .357, the resulting pressure curves and consequently the data from one load doesn't apply to the other.
Using a greater OAL reduces peak pressure in a revolver cartridge given all the other variables remain the same. But reducing the powder density can have the effect of inconsistent velocities. You can also encounter problems where a small amount of powder, especially slow powder, fails to ignite before the pressure from a primer, especially a magnum primer unseats the bullet, possibly even as far as the b/c gap.
Beyond that, it's not clear what your goals are. If you understand that besides bullet mass, bullet type, powder type, powder mass, and primer, the OAL has a big effect on the result, then you might start by setting a goal and fixing some of the variables.
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June 12, 2018, 01:24 PM
