The model 39s I've slugged, have a groove diameter of about .355". However about serial number A200000, the rifling was changed from six groove rifling with one turn in 10 inches to 5 groove rifling with one turn in 18 1/4 inches. Also the lands were made wider at that time. This was said to save time in set up when changing from making 38 Special barrels to 9mm barrels. Without special equipment it's impossible to slug a barrel with an odd number of lands. Did S&W change the groove diameter at the same time, to something like 38 Special dimensions, that is around .357"-368"? And what is the groove diameters of the follow-on 2d and 3d generation 9mms?