Harry's got it. You need to slug the bore. If the groove diameter is bigger than the cylinder throat, then you will likely have terrible leading and accuracy problems with cast bullets and it is likely accuracy even with jacketed bullets will improve from "fine" to "great" by getting the cylinders reamed. Reaming all cylinders to a uniform maximum standard dimension is usually the first step in a revolver accuracy job. Ruger did this free of charge on a Redhawk that belonged to a friend of mine that wasn't accurate. He just returned it to the factory with an accuracy complaint. S&W might do the same if accuracy is a demonstrable issue?
The issue is not pressure; the steel is more than capable of squeezing jacketed bullets down several thousandths. The issue is bullets entering the rifling undersized. This can allow them to tilt off axis or, worse, become more deeply engraved by the rifling on one side than the other. This makes the bullet center of mass off-axis from the center of rotation. Key-holing at distance is usually the result if the problem is serious. With lead bullets, gas cutting around the base is worsened by this size problem. Ideally, lead bullets will almost kiss the cylinder throat for alignment, then be sized down slightly by the barrel. For example: .4300" bullet, .4305" to .4310" throat; .4290" groove diameter. This assumes perfect cylinder alignment with the bore, another thing corrected in revolver accuracy jobs.
A question though: How did you measure the cylinder throats? If you did it with the inside jaws of a caliper, they are typically only accurate +/- 0.002", and can't be trusted on small hole diameters because of the small flats on their edges subtending a portion of the radial arc of the hole, giving a false low reading. You need to slug the cylinder throats just like the bore, then compare the results you get for the two slugs. A micrometer is prefered to a caliper for this so you can see ten thousandths. A gross difference will, nonetheless, show up using a caliper on the slugs. Bottom line, measure both the same way.
Nick