I very much appreciate the responses.

I do have a set of pin gages. They can be useful, and I realize their limitations (provide you with the minor diameter, not the major diameter).

I also understand it isn't absolutely necessary to know the groove diameter. It is useful information to help guide your actions however.

Case 1. I had 3 Ruger cylinders to deal with (2 were 32 H&R Mag, one was 32-20). On all 3 a .312 jacketed bullet wouldn't pass through any of the throats. Using pin gauges a .311" would pass. I checked the grooves on both barrels and they were .313" The Manson reamer I got was to open the throats up to .3135". So I felt I was making the correct action to use that reamer with the commercial cast bullets (.313" diameter) I wanted to use. So at least I knew everything would be want I wanted. It worked out just fine.

Case 2. I have a M14-2 S&W. I am shooting .358" DEWC bullets. They will not pass through the throats. Using pin gages, the throats measure .357". A slug from the barrel passes easily through the throats. I am considering moving to .357" DEWC bullets, but I just would like to know what the groove diameter is before changing. Since the bullets pass through the throats so easily, I wanted to try and measure.

So in either case knowing the groove diameter is just one added data point to help in making a decision.