bear in mind that the 357's extra length over a 38 spec is more a safety function with a ballistic bonus. its not that much longer.
so what you are seeing just might be how the flakes stack.

personally, I'd have built the load around a cast bullet here. on the rare occasion I use a jacketed bullet, you can bet its a full house H110 or 2400 load. its all in the economics.

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it just needs more voltage