> Consider the question then of "at what point in the history of the world could you take three rifles made by the same maker and swap parts and still have it b

> Consider the question then of "at what point in the history of the world could you take three rifles made by the same maker and swap parts and still have it be perfectly serviceable?" It depends on the weapon. To this day, there are many designs that require "fitting" for some parts. The M1911, a .45 ACP semi-automatic handgun that entered US service in 1911 and is still a relatively popular, doesn't have truly interchangeable parts. The interface between the sear and hammer, or the trigger bar and sear, must be manually carefully shaped for feel, safety, and reliability. The "ramp" that guides the cartridge into the chamber is similar, and it's common to get new guns that won't reliably work with common brands or designs of ammunition.