2 Piper, the difficulty in tying down specific gravity of lead and need to go to 8 decimal places are all indicative of why this gauging system fell out of use in favour of linear rather than a spherical/weight/element combination in the latter 1/2 0f the 19c. It is viable for shotguns but once you get into rifled calibers, especially the smaller ones made feasible by smokeless propellants it was found not to be effective.Regarding s.g. measurement metric is the preferred base measure although even this has changed from measuring at basic room temp (60-75f) to a more defined datum of 1 atmos @ 4c. The purity of available lead will also have changed since gunmakers adopted this system.
