Quoted:
I think the real cause was inconsistent powder burn. II think some rounds were hotter depending on where the powder rested in the case (it was only about 1/4 of the way full.)
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I've seen enough reports of 'detonations' / damage from loads like this, that I'm very leery of any load that fill such a small portion of the case volume.
Visualize filling a case 1/4 of the way, turn on it's side (as it would be in the cylinder/chamber) and notice that the powder is all below the flashhole .. it would allow the flash from the primer to hit the entire surface of the powder, rather than the 'base' of the powder.
Rather than burning the powder from the base up, it flashes the whole of the powder at once... burn rate is much faster and pressure spike is much faster/intense as a result.
There is a reason old .45/70 loads that used little amounts of powder called for a filler of cream of wheat or pillow fluff.
I love Autocomp... it's a great 9mm powder. I'd look for something that gives similar velocity with more 'volume' for .38/357 myself.
In the end, you could buy a cylinder and have it fit by a good gunsmith. Heck... call S&W, they might be able to help you out as well. I'd be concerned about frame integrity if that round bulged the cylinder, it may have damaged the frame as well.