Time passed in a post apocalyptic setting

I'm writing a fantasy story with a bit of a post apocalyptic vibe. Up until now, I've been saying that it's set six centuries after the cataclysm that ended civilisation, but it occurs to me that I can no longer remember the rationale for that decision, and the more I think about it the more that time scale seems problematic for elements of the story I want to use. So I figured I'd revisit it here and see what you brilliant folk have to say.

The basic setting is this: a set of nations with 1940-50s level technology are hit by a massive cataclysm. It's not a destructive cataclysm like a nuclear holocaust, but more of a corruption of the environment that makes even short term exposure fatal to human beings. The only safe areas are relatively small pieces of land the size of large towns or cities, referred to as havens, that people desperately tried to get to when the corruption (the blight) started to appear. Needless to say, the loss of life was catastrophic, along with the loss of knowledge and infrastructure. After everything settled down, ways to travel the blight and salvage vital supplies from it (some of which were necessary to maintain the protective qualities of the havens), but the world hasn't gotten any more hospitable in the meantime. If anything, it's been getting worse.

The story I want to tell is well after all this went down, however. What I can't decide on is how long afterwards it should be. These are my requirements:

• Societies need to have acclimated to the changes
• There absolutely cannot be anyone who remembers how it was before... it's best if it's ancient history to them.
• There's a prophecy with obscure origins that predicts the eventual end of the blight. It needs to have had time to disseminate to all the havens and gain a foothold in local folklore and religious practises.
• They shouldn't have run out of bullets yet, despite not having the capacity to make them. I want them still able to use their pre-cataclysm rifles, even if they have to carefully manage ammo and use more primitive weapons like swords, spears, or bow and arrow if the situation doesn't warrant the heavy firepower. Ammunition needs to be augmented by salvage. The maintainence of weapons can be ignored to an extent... they've got some magic that helps keep them in working over for longer periods of time, along with some of their other machinery.
• The salvage of materials can't have run out yet. This is in an area about the size of modern day France, with a dozen havens or so with less than 50K people between them picking away at the carcass of their old society. It's great if the supplies are running low.
• A primitive group of nomads who were relegated to reserves pre-cataclysm need to have had the time to swell their population so that they outnumber humanity by a significant margin. They aren't human, so it's okay if they've got much higher fertility. Likewise, the buffalo-like herd animals they rely on for food need to have had time to create the massive herds that pre-European North America was known for.

Given those parameters, what would be a believable time frame to set my story in? My gut feeling is that 600 years is much too long, and they'd have long since run out of salvage opportunities, been overrun by the natives, or simply overwhelmed by the corruption outside their havens. At the very least, they'd be out of ammunition! I recognize that these are rather fuzzy requirements to work from, but I'd really appreciate some help with this.

Thanks!

150 sounds believable to me. That's a good point about the ammunition... I hadn't realized that smokeless powder munitions were only good for a couple decades. Apparently black powder keeps, though, which is interesting enough.

Black powder does seem ridiculously practical in this instance, so I'm going to have to revise my armanents to have black powder weaponry involved to a large extent. Those can be made into cartidges, but the byproducts of the reaction make automatic or semi-automatic weapons impossible. Bullets and casings aren't too much of an issue either. Casings are no trouble materials wise, and bullets could probably be worked around even if lead wasn't available.

Now, I'm trying to figure out if the production of smokeless powder is believable. I've given at least one of the Havens access to cotton crops, so the real question here is would they be able to make the nitric acid required to turn it into guncotton. Following wikipedia links tells me that the process required is oxidizing ammonia at 850 C in the presence of platinum for a catalyst, producing nitric oxide. After that, it's just exposing that to oxygen to create nitrogen dioxide, and mixing that with water to make the acid. So... the question becomes ammonia and platinum.

If we assume platinum will be easily salvaged from their factories one way or another, I think that it becomes fairly easy to deal with. The only thing I can't figure out is if the Haber process would remain viable. Certainly, there's a lot of impetus to keep it so... the same ammonia that would be useful for creating munitions would be able to fertilize crops.

Would they still be able to make percussion caps, or would they need to find a new kind of primer for their cartidges, or move to old fashioned front loaded weaponry?

Nivarion said:

If they have an outlet to the ocean then they could make percussion caps for their guns. Otherwise, they would have to find a new way to prime their bullets. I think that once we developed cartridges, the idea of a muzzle loading gun like that became so ludicrous that we wouldn't do it again. If they couldn't get any percussion ignited material, such as magnesium then they might make cartridges that had a hole in the base for a fuse, and a flammable material such as paper to cover it.

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The Haven in question is on a river that leads out to the ocean, though I'm not sure that's sufficient. However, there are other havens on the coast, and they could trade with them.

Guncotton, from what I saw of it, was very unstable unless you could treat it with alcohol or ether. If they have a source of those then it could believably be treated to make smokeless powder.

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It would be a crazy world indeed if there wasn't booze available.

One thought, why not disassemble the factory and reassemble it in the haven?

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Main reason is the aforementioned blight. It can be traveled safely if you pack the right equipment, but it's very, very expensive to stay out there a prolonged period of time. The number of people you'd need to involve to dissassemble a factory in the blight then haul it back would be impossible... you simply can't shield that many people at one time economically.

Fortunately, the haven in question was a trade hub pre-cataclysm, so I would not be surprised if many of the relevant factories were within city limits.

There are things out there with sufficient motivation to kill them. Including their haven-mates, on occasion. Interhaven war doesn't happen just because the logistics of it are beyond any of them, but there are other threats (for instance, the aforementioned nomads) for which the additional firepower is always appeciated.

Unfortunately, the same reason that war isn't viable rules out the kind of large scale salvage operation you're talking about, Nivarion. It's not just a matter of losing a lot of lives, they simply can't protect that many people at once. And anyone without protect will be dead in minutes, not the days or months required for ripping down and moving a factory.