This goes back to an HS-6 thread from January 2011 concerning .38+P loads. (HS-6 in 38 Special?) Just so there is no misunderstanding, I am quoting from a post on that thread made by ArchAngel, but I am in no way being critical of his remarks.

"Hodgdon lists a charge of 7.0gr with pressures of only 15,500 CUP in .357 Magnum brass using a Magnum primer. Granted the Magnum case is 1/8" longer but it's hard to believe you can increase the charge weight that much and have such low pressure from only 1/8" more capacity."

I am referring to his remark which is underlined above. There is a common mis-understanding that because there is 1/8" difference (Actually .135") that the relative overall lengths of .38 Special and .357 Magnum reflects their relative case capacity, it does not. Using Winchester cases and the same bullet this is how it really works.

It would appear, based on overall case length (1.155/1.290=.90), that the .38 Spl case has 90% of the capacity of the .357 Magnum, but it really doesn't. A closer comparison is the length of the "combustion chamber" between the two cases. With a seating seating depth of .400" the distance from the inside of the case head to the bullet base is respectively (.598/.721=.83), making the "combustion chamber" volume for the .38 Spl. case 83% of the .357 Magnum.

Using this comparison when converting a load from .38 Spl. to .357 Magnum, if you increase the .38 powder charge by 17% you will get approximately the same pressure and velocity in the .357 case with all else, primer, bullet, barrel length, being equal. For obvious reasons (I hope!!!!) this does not work in reverse.

Now, something to think about. A commonly recommended load for .38-44 equivalent loads is 12.5 gr/2400 and 358429. There are several things wrong with this if you make the same measurements and calculations as above. The top .357 load listed by Hercules/Alliant for .357 is a 158 gr LSWC (lighter than the 358429) over 15.3 gr./2400. Disregarding the bullet difference and just comparing case volume using the above figures, to get the same pressure in the .38 case as in the .357 the powder charge would have to be reduced by 17%, or to 12.6 gr with the same bullet. The 12.5 gr./2400 load in the .38 Spl. case will be producing approximately the same pressure as the 15.3 in the .357 case, or 45-46,000 CUP! Do you really want to be shooting this sort of pressure in your .38-44? Let's go farther. Since S&W advertised that factory .38-44 ammunition was acceptable for use in pre-war M&Ps, would you really want to shoot this load in a K-frame????? What I am trying to say, is that this is a MUCH HIGHER PRESSURE load than the factory .38-44 was in the 1930s.

__________________
Gunsmithing since 1961