Two questions:
Why should one care about realism on a game with talking chimps, psions and other mutants?
Without an industry to melt/forge/machine the guns how do you figure those would make a comeback?
I think it is more about consistent world building than realism, are the consequences believable and if they vary from expectation is this explained in the lore. Talking chimps, psions and other mutants fall under willing suspension of disbelief, we don't have any in real life so accept the author's word that they work as described. When we do understand how something works, deviating from that pattern requires a justification of some kind to keep things plausible instead of nonsensical.
Firearms would produced by cottage gunsmiths instead of industrial manufacturing and would be made from easier to work materials like brass and iron instead of steel, aluminum, or polymer. Weaker materials would be suitable because of the lower pressure of black powder. It's not a net energy problem but a pressure curve, fowling, and clouds of vison obscuring smoke problem. I can elaborate further if such would be of any value.
That's fine and dandy, and I would asppreciate further elaboration.
Now, here's the thing: How many people KNOW that gun ammo deteriorates over time if not correctly stored? Not that many I would guess, we know that guns withstand centuries even under sub optimal circumnstances, so, finding guns that work is a throw of a dice (two really), same for ammo in good state.
Smokeless powder fabrication is very involved and requires several previous steps. In the novel in my head I have a perfect explanation as to why some settlements have the means to elaborate it. This might end up as part of the lore, depending on which way I end up going.
But, again, modern revolvers do work with black powder since they don't need to move the carriage, etc. The range would be less of course, but I already have the range for Western weapons.
Let me expand a bit on the finding relics part:
The party goes to a city, it's overgrown with vegetation and full of mutant plants and beasts, maybe even some intelligent gator men. After avoiding/killing/whatever the oposition they find a police precint, go inside and find the cops weapons (some of them) then they find some ammo, roll to see how much and if it's in good state (or the GM does this when prepping).
Then they go to a museum, turns out it has some ancient vehicles (I know one such here in México city specialized in classic cars), well the tinkerer goes and makes sure one is in good enough condition, but there's no battery, now what? can they find the stuff needed to build one?
Or they find a steam tractor, well that's "easier" just make sure it's in good condition, fill it with water, find some wood/coal and you have a working vehicle that can drag the muscle car (or several wagons full of scraps they found).