Since this sort of thing honestly seems to happen a lot in RPs like this then I'd like to say something while this is still in its infancy, so we can at least discuss the trope or the cliche in such realms. This does not mean I am disinterested, I am in fact starved for a good NRP and this will have to fit the bill. But that topic is: guns.

Really, the fundamentals of fire-arms design isn't all that complicated and even after 600 years of difference between today and the time this RP is set during fire-arms probably wouldn't be phased out. They'd evolve for sure to meet the economic demands of a post-apocalyptic, politically decentralized/devastated part of the world but they probably wouldn't be gone for good. And that's because the fundamental process of building even a rudimentary fire-arm is very simple. In fact, if a society could manufacture a cross-bow they'd be able to manufacture a single-shot rifle/musket.

And basically the pipe-built guns of Fallout 4 isn't all the unrealistic. The fighters in Chechnya have been constructing crude pipe-based guns to use against the Russians for a while now.

[img]https://img.buzzfeed.com/buzzfeed-static/static/enhanced/webdr02/2012/12/20/13/enhanced-buzz-3544-1356026577-4.jpg[/img]

[img]https://img.buzzfeed.com/buzzfeed-static/static/enhanced/webdr02/2012/12/20/13/enhanced-buzz-3637-1356026669-10.jpg[/img]

So really, even after modern manufacturing dies the art of even the most crude gunsmiths could survive on using just the debris left behind and would be maintained after. The basic formula of gunpowder isn't very difficult at all, it's a simple three-ingredient compound using rather common components: carbon from charcoal or coal, salt-peter/Potassium nitrate from fertilizer or even [url=http://www.todayifoundout.com/index.php/2013/04/urine-facts/]urine[/url], and sulfur mined from regions of volcanic activity or extracted is a byproduct of regular ore mining (but, I also refuse to believe that even after 600 years alternative processes for anything would be simply forgotten and not passed down into being folk-way methods).

Mixing these at ratios of 75:15:10 (Potassium-Nitrate:Charcoal:Sulphur), and wetting down with water to prevent the early mix from combusting during initial mixing will create rudimentary black powder. Black powder doesn't pack the same amount of force as today's smoke-less powder, which means any bullets fired from firearms using it will have a lower velocity that normal and shorter range but can still be used in a gun.

After it's just a matter of rigging a barrel to a shoulder stock with a tortion-operated trigger mechanism to ignite a flint, strike a smoldering rope to a flash-pan, or hammering against the back of a bullet to ignite a percussion cap to initiate the detonation of the powder and firing the bullet.

Economic conditions may vary the capability of and effectiveness of the gun in question which can vary between regions. But that's the real issue at question: the effect of regional economic ability and the extent a society can reach within the limits of technology. Clearly computers would be dead for reasons, and the total collapse of the modern metalurgy industry will destroy the capacity to produce a lot of heavy industrial metals. But that's all without saying. But none of this should destroy the capacity to make single-action rifles/muskets.