>I'm considering adding an Armor stat for enemies, but it's hard to working with that mechanic without making any non 5.56/7.62+ caliber weapons useless.
Sounds like a nice progression mechanic right off the bat. :3
What about playing with weapon accuracy and enemy weak spots? SMGs would be ideally more accurate in full-auto, able to send a tight long burst right where you need it (for example somewhere armor doesn't cover). I see the apocalyptic theme allows for big ass mutants and bosses so this could be somewhat feasible. Rifles on the other hand could be more accurate in short bursts. Have you thought about a CS-like bullet spread? Long full-auto bursts would send bullets over the crosshair in an increasingly erratic pattern, more so with rifles and less so with SMGs.
And finally you could think about playing with exaggerated ballistics as to give different weapons their own feel and purpose, video related.
Sorry to project my expectations and autistic ramblings over your project, but I love detail and I'd definitely drop a few bucks on it if it featured some of this stuff. Similarly to your philosophy about weapon degradation it could affect gameplay but not cripple it, making it much more interesting (to autists like me).
Definitely play SOLDAT if you haven't, it has great 2D weapon feeling.