FloridaFlier, Measuring the five groove rifling found on the S&W's is quite difficult. As you noticed, the grooves are not at 180 degrees, so getting an accurate micrometer reading takes more than average skill. Any how, in revolvers, the cylinder throat size is more important than barrel groove diameter. Getting your mold or sizing die to as close as possible to cylinder throat size, with a good bullet lube, will usually get you the best results.
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