Well, as demonstrated by the picture above (micrometer) and the methods used to wrap the slug to measure one with an odd number of grooves, it is difficult to get a precise measurement. What I was hoping for was a comparative based on the internal bore (land) measurement. Without buying the expensive micrometer, I am not sure I have the correct diameter by cutting up a beer can and wrapping the slug. Using that method in my 44 Spl I come up with .429 for a groove diameter but using a pin guage thru the barrel I get .416 which is the bore, or land, diameter. A groove depth of .0065 seems like it would be too much so I am concerned I have an error. The obvious place to look is the least precise measurement, the groove diameter of .429. That is the background to my question.