Raimey, when you speak to him, some considerations- The number on the barrel flat is measured by pin gauges that are in 0.1 mm, so a "slug" measured with a mic might be a couple thousandths of an inch off that dimension. If the barrel has an od number of grooves, measuring the groove diameter with common tools can create an error, using various methods to approximate the grove diameter of an odd number of grooves can also result in an error, albeit a smaller one. Old barrels often have "tapered grooves which results in the groove diameter at the breech end being a couple to a few thousandths of an inch larger than at the muzzle end. All these things should be considered.
Mike
