Interchangeable parts is something we take for granted today, but it wasn't always so. Some pioneers in the 18th Century showed the way forward, but had plenty of opposition to their ideas. Gunmaking was a highly skilled craft then, and the gunmakers themselves were not exited about giving away their livelihood to a bunch of machines.
Honoré le Blanc was a French gunmaker who realised that if guns were made to exact tolerances, a part from any gun of the same batch would fit all the other guns. In the 1750's this was absolutely not the case, as any part from another gun would never fit, and needed extensive fitting by experienced gunsmiths, making gun repair an expensive proposition.
The French gun guilds had a lock on gunmaking and gunmakers, which besides their livelihood, conferred status and power upon it's members, and their power wasn't broken until the French Revolution. There was an underground in gunmaking, but to begin a concept of interchangeable parts, the guilds would definitely have to be brought onboard, and they weren't having any of it.
Of course, there were people around who liked the idea of making and repairing guns without the having to resort to European gunmakers to do it, like Thomas Jefferson. His newly minted country needed a homegrown gun industry immediately, with no time build a huge European style gun guild. Thomas Jefferson visited LeBlanc's workshop while Ambassador to France and witnessed the advantage of his system of manufacture.
Thomas Jefferson, while Ambassador, was unable to convince his country in this new manufacturing method, and couldn't persuade LeBlanc to move to America. It was left till Eli Whitney came along, intent on manufacturing guns on the new method for the United States. Officials dragged their feet, but Thomas Jefferson, the new President, backed the idea. Fifty years later, old Europe was making guns on the American Method, one they had thought of years earlier but disdained.
The debt the Americans owed LeBlanc for helping to show the way forward would be repaid many years later when the Arsenal of Democracy liberated Paris from her tormentors in World War 2.