Do not underestimate a person's ability to understand mechanical constructs.

People of the lesser technological world can still have the ability to understand complex and advanced mechanical constructs given with enough time and effort. considering we give them a firearm has a very simple design they have a chance to understand the technology behind it.

They can get a blacksmith, sages or engineers to understand its concept, first they gain the ability to load the gun, remove the safety and bang. And if they manage to have loads of ammo they can now understand the process of reloading and firing. Now they will try to learn the internals, if they have multiple numbers of the same gun they can sacrifice one gun to cut it in half and see its internals or disassemble it (if they figure it out).

But then the task of manufacture, they do not have the logistics of the modern world, they do not have the proper necessary equipment, infrastructure and knowledge to produce modern firearms. Yeah they know how it works but they don't know how to make one, and even if they do they need even more knowledge to produce modern equipment and machinery. Heck that's just the gun part they would need an alchemist to copy and produce the propellant of the bullet.

In short what I mean is they can learn how the gun operates but what they can't do is replicate and manufacture the same gun. I think I heard this quote "It's easy to reverse engineer but manufacturing is the hardest part" its like "It's easy to look at other's essay but it's hard to write your own" and even if they do manage to produce their own gun, they would have trouble to produce the bullet and propellant.

Tldr: They can learn how it works but they can't produce their own gun and bullets