Perhaps it's possible to teach the smarter monsters (ie not zombies) to seek cover when faced with guns. This would let you make guns realistically accurate in skilled hands, but not all-defeating.
That's certainly something to look into, but it doesn't solve the main issue of defeating all the zombies trivially.
I also think you might focus on "end game" players a bit too much? I haven't played cata very extensively, and the characters I've had have never really found usable guns. I'd like to think that if I'm a survivor just working with what I can get, finding a working gun with ammo should be a big thing.
No context on how early game you are, are you making it into towns where you can loot shops that have guns, or are you just exploring the outskirts of towns? If it's the latter, that's pretty much working as intended, guns are supposed to be very strong (and despite complaints about effective range, they *are* very powerful within that range), so they tend to spawn in relatively dangerous areas.
Options for reigning in gun power once ranges are extended:
Hordes attracted to gunfire is the top contender, but that's been stalled for quite a while, we can't count on that for a short-term fix.
Harder to kill enemies is pretty promising, but has shortcomings depending on how it's done.
Enemies taking cover only works for smart enemies, and is gated behind meaningful cover as a game feature, so also not short-term.
Enemies with more HP is somewhat promising, but is also kinda gross from a ui point of view since the aiming interface isn't particularly fast, it needs to be streamlined before making monsters soak significant numbers if bullets is a viable option.
Giving enemies hard-to-hit weak points (shoot zombies in the brain, shoot regular animals in organs without hitting bone) seems like an interesting middle path, making the target you have to hit effectively much smaller while leaving guns more reasonably accurate. Ideally this would scale differently per monster type, zombies might take almost no damage if it isn't delivered to vitals, natural animals would be susceptible to shock, and therefore more vulnerable to not-perfect shots. This also has the benefit of trivializing encounters once a player becomes overpowered for it, unlike high HP across the board.
Making guns/ammo rarer.
Reducing numbers of guns/ammo across the board is a pretty hard sell outside of a "UK mod" or similar, the current reality is that the setting (USA!) is absolutely saturated with guns and ammo.
What *does* make a lot of sense is positing that most easy-to-loot guns, and especially ammo, have been, leaving most off the ammo concentrated either in difficult-to-loot areas or possessed by NPC factions.
Hope that clarifies some things.