This shows the difficulty in trying to determine the nominal caliber (name of the cartridge) from proof marks and chamber casts. The proof marks do not, themselves, show the nominal caliber of a rifle, rather they show some of the dimensions of the chamber. These dimensions may be unique to one nominal caliber but are sometimes the same for more than one. An example of the same proof marks used for more than more than one nominal caliber is those for 8x57 I, 8x57IR, 8x57R/360, and 8.25x57R Hagen (AKA 8x57R Express, AKA 8.2x57R-360 Express). The problems with chamber casts are that they do not duplicate the dimensions of the cartridge, but of the chamber. The chamber must be larger than the cartridge, otherwise the cartridge will not chamber. How much larger? That is determined by the maker of the rifle and may vary from .005-.015" depending on the size and age of the cartridge. Chamber reamers are made with a + tolerance and cartridges are made with a - tolerance (again, to ensure any commercial cartridge will chamber in any rifle of the same nominal caliber). Nevertheless, except for rifles made after the 1939 proof law mandated that the nominal caliber be shown on the guns chamber casts and proof marks are usually the only information, we have to identify the nominal caliber of a rifle "in hand". To this information we have to apply previous experience and knowledge. For instance, we know the bore diameter shown by the proof mark is not the groove diameter of the rifling or the bullet diameter of the cartridge. We also know that older, black powder, cartridges usually used bullets smaller than even the bore diameter and depended on obturation to make the bullet fit the barrel (these were often made before proof marks were used). With all these considerations, identification of the correct cartridge is still a matter of judgement and mistakes can be made; but even known mistakes represent additional information for reconsideration of the final identification.
Mike
