In my story, a mysterious visitor presents the characters with a new weapon: guns. These people have never seen anything like it, as gunpowder has been discovered but not really used to its full potential. So they know what gunpowder is, as well as metal. However, they do not know about things like bullets or triggers or barrels. So how would these characters describe these gun if they have no knowledge of them if I also want to give clues to the reader knows what they are before the character does? Note that I am writing in thrid person and in the present tense.
You should try to draw comparisons between the gun and the things your characters already know. Try decomposing the gun to its most essential parts and describing those.
My solution to this specif case (a revolver type pistol) would be something on the lines of "a small metal object, looking like the hilt of a sword, with a lever which produced a loud bang when pressed". If you want, describe how this object shoots "magical small metal arrows" and uses "black smelly earth" to function.
The description depends on the point-of-view character, the actions of the gun-wielder, and the current weapon systems available.
If your POV character is observant, quick-witted and capable of making inferences you get a different description than if they are arrogant with a sense of cultural superiority.
The physical description is couched in terms of what is familiar to the POV.
The strange men hastily formed an outward-facing circle. Each quickly drew a short, hollow cylinder from open pouches at their hips. They raised one in each hand towards the warriors, shifting the target of their flute-like devices to the warrior closest to each of them.
The action of forming a defensive circle and raising something is a clue to your readers.
You can also describe the way they sight down the barrel, or they way they move the barrel between targets like a bow and arrow.
The chief understood the strangers believed their devices should keep the warriors at bay. These were obviously weapons of a sort. But were they actually dangerous or merely a superstitious device to send evil spirits towards the warriors?
Once the gun is fired you can describe the sound, small and sight of the gunshot plus the effect of the weapon on the target. You can also describe the changed demeanour of the gun-slinger. But that's getting beyond your question.
Human males generally posture and scream before resorting to violence, if this is a first contact type story.
You write that gunpowder and metalworking have been discovered. So the people of that world know that gunpowder explodes (and that explosions create a force that propells stuff) and they know objects made from metal such as tubes (and if they don't, they know tubes made from other materials such as clay or wood).
Given this knowledge, I do not understand the difficulty in describing a gun and why you would accept an answer that compares a gun to a sword hilt.
If the information given in your question is complete enough for us to answer your question, then the following description of a gun should be appropriate:
A handle attached to a metal tube, through which a metal ball is propelled by exploding black powder.
Note that instead of "gunpowder" – a term derived from a word for an object that does not exist in your world – I use the synonym "black powder".