When old guns were made they didn't work without significant amount of craftsmanship: machining, measuring, fitting, polishing, etc.
New guns are made mostly on automated machinery with little human intervention and a calculated percentage of warranty work to be done when they leave the factory.
Eli Whitney was the first person to build guns with standardized parts. I'm pretty sure Colt and others continued that practice.
I agree that manufacturing technique is as good a dividing line as any. Perhaps a bit more specificity would be in order.
A lot of technology is adapted well after it was physically possible. Steel strong enough to make a handgun capable of high pressure ammo dates back at least to the Bessemer process of 1840 and probably much earlier than that. The difference between the 1840's and 1900's was more quality control.
Cartridges were possible as soon as fulminate of mercury was developed in 1800. The difference between striking a stone against steel and chemical reactions is a bigger difference than moving from charcoal and salt peter to nitrocellulose.
The way we shoot changed a lot then too. Not having a gout of flame shooting up in your face made aiming lots easier.
Just some random thoughts.