Cast bullets are most likely to be accurate in a revolver if their diameter is matched to the cylinder throat diameter, not the groove diameter. With one exception S&W centerfire revolver barrels have five grooves so it is difficult to measure a slug driven through them. For those two reason very few of us measure our S&W revolvers' groove diameters.

The exception is .45 ACP barrels made before S&W changed from broach cutting rifling to using electricity to erode metal away to create grooves. The change was about 1997. For the first 90 years that S&W made .45 ACP revolvers they used army specification 6 groove rifling.