[SDC]

The history of proof marks is a study unto themselves, and there are a number of books on them; part of the problem is that each country (and sometimes proof-houses WITHIN a country) instituted their own system, each with their own accepted level of overcharge, type of powder used, weight of projectile(s), and so on. The "Nitro" proof dates from the realization that nitro powders placed a much more sudden and sustained load on a gun when it was fired, so they had to be proofed additionally specifically for those powders; the usual standard was at least a 30% overcharge, and if the firearm took that and didn't show any ill effects (no warping or measurable changes of the chambers, barrel, or frame), then it was assumed to be good for any standard load of that calibre/gauge. In the case of a revolver, they'd fire one proof load from each chamber, inspect the revolver from top to bottom, and then stamp it as having passed.