This is a problem of making your guns out of plastics... There are plenty of good "old school" guns which are reliable, durable and simple to handle and operate.
At some point the survivors certainly will start to re-manufacture the stuff, which will lead to further development. But i dont really see a way - nor the need - to model the development.
In real terms, the big difference between the firearms of the pre-ACW era as that you pretty much had to be standing up to fire them since you had to use the ram (and cleaning) rod to load the weapons. The advent of things like the Spencer rifle allowed people to be able to be more flexible in using the terrain. Not that you couldn't do it before, not very feasible if you have to get up every time to reload.
The second big advance didn't come to about the time of the Boar War when the introduction of 'smokeless' powder was introduced. You can always tell people who have no concept of early firearms when they smugly think the British were 'so stupid for wearing those bright red coats' when the fact of the matter is EVERYONE back then had uniforms that had some form of bright color in them because after the battle got going, you needed that color to tell who was who. Anyway, so in an ambush, you'd get one volley and then the smoke would give your position away as telling as waving a flag. The 'smokeless' power gave the defenders a big boost since now, unless you saw the muzzle flash, it was much harder to tell where an attack is coming from.
Now, a typical ACW rifle could still out fire a crossbow, had more stopping power and those wounds were hard to cure back then. Plus, their rate of fire was still pretty impressive and as I've posted before, they would not be hard to make.
Considering the firearms that would be made, the big question is when can they make a reliable cartridge? Because in many respects it is not the soldier level weapon that is important, but the belt-fed squad automatic weapon that is going to be the big one, especially since they would be much more effective on the more modern vehicles then they were originally when they were first lugged around like artillery.
So in game terms, getting the resources to make gunpowder, building the facilities to do so (especially since making it is VERY dangerous) is the big stumbling block of firearms. Once that happens, it's just a matter of being able to reach a machining tech of around 1900 or so when you can build the cartridges for machine guns.
As I've also said before, I believe ammunition production would be the biggest and most continuing problem for the survivors. Even when you get your gunpowder going, it'll take a bit to build enough ammo plants to field a lot of units. Then when you get to the next step, you'd need an 'advance ammo' plant to crank out even more ammo, even more so because the belt fed/magazine fed weapons shoot more ammo and their would be more soldiers by the time the tech is online so you'd need even more.
In game terms, it's too bad there probably isn't a way to track units with a certain promotion, since it would be nice to be able to limit the number of units with a certain promotion to the number of something like amount of ammo plants built.
Last thought guys (sorry for long post) Unlike vanilla Civ where it's common (and stupid) to see spearmen long into the gunpowder age, the scale of this mod and the scarcity of resources might be that you'd still see lot of the early tech units simply because there are not enough resources around to field everyone. We must not forget there is a big difference between a modern nation (even small ones like Portugal or Holland) could produce from 1812 thru 1917 compared to an area wracked with climate change, constant warfare and the like. Not everyone can have the good toys.