Guns, gunpowder and artillery exist, and their existence is common knowledge throughout the world. The most prevalent firearms are cannons, used in ships and on castle fortifications. Handheld guns, though, are much less common. This is because the chemical reaction that ignites the black powder used to fire ammunition from the a gun is far too corrosive to iron for it to be useful. For a gun to have a barrel thick enough to not be worn away after only a few shots, it would be so heavy that it could not be carried by a single soldier.
However, since the cost of a single cannon is already large, making the barrel of a cannon thick enough is an expected cost; also, part of the reloading process of a cannon is coating the closed end of the cannon with tar – too much for it to be feasible for a soldier to carry for a handheld gun.
Despite the high cost – and near infeasibility of iron guns, the dwarves of Budynsky have an almost plentiful supply of handheld firearms. They have achieved this because of a discovery made at the Umyswoveh research facility, that gunpowder is not corrosive to adamantine, a rare metal found only in the Budynsky mines. Because adamantine is only available to Budynsky dwarves, they have been able to keep the secret of their highly efficient guns from the rest of the world. Some, however, have slipped into the rest of the world, but non-dwarves have never been able to reproduce them, due to the lack of adamantine outside the world.
If a character has a hand held iron gun, it will be able to fire two shots before gaining the fragile condition, and two more shots after that before the black powder corrodes the barrel and it gains the broken condition – the barrel of the gun literally falls off of the stock as the pressure from the last shot shatters the rusted base. If a Make Whole spell is cast on it, the gun will be able to fire one additional shot before corroding and its condition will improve if the new amount of shots fired would require a lighter condition, or if it was broken, it will be made useable and able to fire again, albeit with the broken condition. An iron gun with the Impervious magical property, it will be able to fire without regard to black powder’s corrosiveness, as if it were adamantine.
Chemistry note: I know that in the real world black powder is only somewhat corrosive to iron, otherwise the development of modern military history would have been a lot different. However, in this fantasy setting, the corrosive properties of black powder are much more dramatic, so much so that they make iron guns unreasonable.