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The cartridge case on a .38 is shorter than a .357, so you can get some fouling at the front of the chambers...nothing a good cleaning won't cure.
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I would agree, its possible some throat erosion on the front part of the cylinder as the 38 is a bit shorter, this is possibly where this comes from.
But you'd have to run a lot of rounds though it to see anything.
Probably more rounds than even a serious shooter would go though.
Nothing I would even worry about though.
I probably run thousands of .38 rounds though the S&W 686 shooting steel matches, and still used the gun with .357's for deer hunting, never had any problems.
imho the problem with practicing with a light gun and high power loads is the flinch that usually comes with it, using the reduced loads helps stop that...usually..
Al