"It is not feasible to hold on to area's with low populations" possible or not it's an unavoidable necessity, if you don't hold onto that low population density farmland everyone in your high population density cities starves to death // I guess you're not planning on a long story arc then? :)

The canonical answer is pump-action shotguns. Why doesn't it work for you?

@AlexP because that is what I suspect it will be, but like I said a suspicion does not mean its a reality. And who knows, perhaps there are multiple weapons that might fit the bill.

Monster kebab it is then :) what wine do you recommend with that?

@Pelinore it depends. A spider bear● goes well with a smokey Pinot noir while a Cthulu sushi■ goes well with a sauvignon blanc. ●images.app.goo.gl/JajpF2jpkDcdxu6w6 ■images.app.goo.gl/yVNwV6jrXvM9qhXAA

@Demigan: You can't farm enough inside a city to come anywhere near feeding the city. As for eating monsters, all well and good, but what do the monsters eat? Unless they're just popping in from a different dimension, it seems you'd be in a situation equivalent to folks hereabouts trying to survive on bear & mountain lion.

Sorry if your enemy can appear anywhere, any time, in any numbers, your defenders are toast regardless of what weapons they have. I get the sense that wherever these nightmares come from simply doesn't care about losses, trading 10000 nightmares for a single human is perfectly fine. But say they can simply appear, you have guarded sleeping quarters and one appears sitting on Fred's face, starts munching. Even if your guard manages to kill the thing you've still very likely lost Fred. That simply isn't sustainable. Now, if you can create some kind of exclusion zone that would be very different.

@Jamesqf yes they come from another dimension. Even if they didn't, what is the problem? These are nightmare creatures that can apparently teleport into your bedroom, physics of the creatures seems less of a problem right? Its a bit like complaining Star Wars has the Force and the potential for infinite energy. That is not really the focus of the story, its a space opera!

@SoronelHaetir you assume that there is sentience behind the nightmare creature attacks, and that the story is about stability. The story would actually incorporate the slow degradation of society and incoming doom as people die off faster than children can grow up as part of the narrative. Its not a problem for the story.

@SorenelHaetir also if you like to know, the fluff has as mentioned a few minutes warning before the creatures arrive. Fred will grab his gear and hide somewhere else after warning the militia and neighbours rather than wait for something to spawn on top of him and get eaten. That is no guarantee for safety, hence the requirement of a self-defense weapon. But it does cut down the speed at which people die.

This isn't an answer exactly, but most of these conditions apply to actual insurgencies. If you were to just remove the ammo requirement, the AK-47 would be an obvious choice. I don't know how to do this myself, but I don't even think making your own AK bullets is that hard. I'd seriously consider this weapon.

The same gun Russians armed civilians with, the Mosin Nagant, the third most ubiquitous firearm in the world after the AK and a musket, which fits all of your criteria except firing makeshift ammo, but that doesn't matter because there's still 'spam packs' left over from WWII, let alone the fact that 7.62x54r is "increasingly available". My family owns two; one with a sniper bolt. They're $100....

@AlexP I don't know why the shotgun would be canonical. It's not like the widespread arming of civilian populations has never happened before. I don't know of any instance where shotguns were the favored weapon.

@Mazura Where can you get one of these for $100?

@user37344 in those instances, ordinary ammo was still widely available. In my scenario the civilian weapons were build en-mass with the idea in mind that they would need to fire makeshift bullets at some point and maintained by inexperienced hands

Makeshift ammo would likely eliminate most existing modern firearms as possible answers, as firearms tend to work with specific bullets, and only those bullets. If you can compress and shape the ammo into the shape of a bullet, then this requirement may not actually change that much. It would still need to be hard enough to survive being fired from a gun though. You're going to have a hard time firing soft or fine things like dirt or most of a dead body out of most guns, and, if you manage to, doing any significant amount of damage would also be a problem.

Suspending disbelief about the fantasy aspects is one thing, but, the world ended and there's no severe psychiatric problems?

@Demigan: If you don't want some degree of reality in your world, why are you asking for answers? Just do like those Star Wars [explitives deleted], and construct your world from pure handwavium? In a reality-based world, if monsters can be eaten, they also have to eat, and so are subject to the same food chain conditions as any other creature.

@jamesqf I do ask for a degree of reality, however there are fantastical elements. The question handles weapons for civilians, not a food problem nor how&why the nightmarish creatures survive. So why are you commenting about it? I already mentioned that a question about food is already a seperate question. You are essentially complaining that Star Wars has sound in space so you ask how planets can exist if there is apparently an atmosphere everywhere. But that isn't the point and not even how they handle the world. Frankly I think you just want to find a problem rather than there being one.

@Demigan what is "short range" in this context?

@CarlKevinson anywhere between the corridor of a flat (say 40m) and AAAAH GET IT OFF MY FACE