What is the prefered way to measure the 5 groove S&W rifling? I tried two methods. First pinching a piece of thin shimstock around the bullet, then measure the diameter, and subtract twice the thickness of the shimstock. The resulting figure will be your diameter. Secondly, place a bullet or pin gauge of known diameter in a V block. Measure from the top of the bullet, to the underside of the V block. Deduct the diameter of the bullet from this figure, and write down the difference. This will be your base measurement. Place the bullet to be measured in the V block. Measure the total from the top of the bullet to the underside of the V block, and deduct your base measurement. This will supposedly be your bullet diameter.
On my 16-4, 32 mag S&W, I got the same measurement, using both methods. I got .309, which I am having trouble believing is correct, as it is so small. My cylinder throats measured .311, with a pin gauge, so I know that is correct. At least the cylinder throat to groove diameter relationship seems to be correct.