Quickie check, Try puttin fired brass from the GP into the SP.
Nuther thing.....measure the chamber throats...at the barrel end. Could be that the SP cylinder is bored tighter. Quickie there is see how far a jacketed bullet goes in from the front of the cylinder.
And, I have found a .009" spread tween tightest and loosest .357 barrels. On new guns.
Sam
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