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For a setting that I design I deal with REBUILT post apocalyptic weapons.

Sooner or later someone (possibly more than one side) would try to manufacture some weapons. Actually, not just repair existing guns or create some more or less fitting ammo, but provide a few hundred (and in long run thousands) members of some paramilitary with new, standardized guns and ammunition.

Realistically there should be some incomplete blue prints, working or damaged examples, partially complete information about how the weapon should work. Reinventing the wheel is not necessary. The guys hired to do the job are quite bright, some older members even had formal schooling before the apocalypse.

Assuming that there is no need for backward compatibility, to what extend should they realistically copy existing guns (with minor variations caused by initial quality problems)? Or maybe, while reverse engineering deciding that such an old design indeed was quite good, but slightly adjust it to local needs (like different caliber, barrel length, etc.)?

Under normal conditions there is a pressure to keep standard caliber because of logistical needs and compatibility issues. In such a case that would no longer be an issue. Would it mean that at such a moment it should be adjusted? (Based on local conditions, materials accessible at that time and the whim of a warlord... ehm... local military doctrine.) Or maybe with limited resources there would be no choice but to produce (imperfect) clones of existing weapon?

I am especially interested in calibers. Because they are often something that sticks, even after the initial reasons for adoption changed. Should calibers be kept during reverse engineering for simplicity reasons, or would they easily be adjusted based on the needs of the specific situation?

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    $\begingroup$ Which calibers? Do note that there are a lot more of them than the bog-standard 5.56/7.62/9 mm. In general though: Chekhov's Gun applies... or in this case I suppose: Chekhov's Ammunition. Is it important to your story? Does it provide any additional story hooks, plot flavouring, meaningful descriptions to flesh out the air of the setting? Only dwell on this if it is meaningful to your narrative, otherwise leave it unsaid. $\endgroup$ – MichaelK Apr 28 '17 at 11:21
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    $\begingroup$ Whichever is easiest to fabricate will be your caliber. If you can repair some old gun fabrication device, you'll use that. If you start from scratch, the size of your best drill, or a bunch of pipes you have lying about, etc. is what will determine your new caliber. $\endgroup$ – Swier Apr 28 '17 at 12:45
  • $\begingroup$ How serious is the apocalypse? Right now if civilization fell apart, there would still be guns, gun parts, ammunition, and ammunition components all over the world for decades. Reusing existing calibers would make sense if, for example, they happened to have a truckload of brass cases or barrel blanks for a specific caliber. $\endgroup$ – Deolater Apr 28 '17 at 13:08
  • $\begingroup$ Hum, Calibers .... It is just a word used to label a Standard. No one is required to adhere to them now, in the past or the future. People adhere to them because they want to have access to the groups that use that/those standards. Should your Future World choose to NOT adhere to these standards and the Word Calibers, you just create a new word and a new standard that does exactly the same thing. $\endgroup$ – Enigma Maitreya Apr 28 '17 at 13:12
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You need to determine where you story is set: America, Europe, Asia? This determines your caliber because there are machines to produce 5,56, 9mm, or 7,62 that are most common to find. The caliber then determines your weapons. It's better to have guns and rifles that can use the ammo you produce.

And forget the, "Oooh it's the apocalypse, we magically have only scraps of 150 year old knowledge (that have been running through these 150 years.)" It's bad and rubbish. Making a gun is as simple as putting a spring with a needle on a stick. My grandfather's brother lost an ear that way when he built his own rifle for a 9mm he found after the Germans left.

If you have a metal press and a sheet of metal you can make a gun.

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    $\begingroup$ Your argument for how easy it is to make a gun is that your great uncle lost his ear trying to make a gun? That's not the best argument... $\endgroup$ – Azuaron Apr 28 '17 at 12:51
  • $\begingroup$ He was 7 or 8 years old back then. I have a little hope in humans but I think a tad older person could do it without losing any parts of their body. $\endgroup$ – SZCZERZO KŁY Apr 28 '17 at 12:55
  • $\begingroup$ Making a homemade gun for existing ammo is easy. This one kid on youtube has made all sorts of stuff of varying sophistication. Here's an example youtube.com/watch?v=NVhceWZiYPQ Making high quality ammo from scratch is a different game entirely. $\endgroup$ – Deolater Apr 28 '17 at 13:06
  • $\begingroup$ Making a homemade gun for homemade ammo is also very easy. And we are talking some sort of industrialization. I don't like the idea of "reverse engineering". You would rather start from something like XVII century ammo and then just do a quick research how much you can jump ahead. $\endgroup$ – SZCZERZO KŁY Apr 28 '17 at 13:31
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Many modern items are built out of things that are themselves built. A bullet is assembled from multiple components. Each component itself has a supply line, required machining tech, metallurgy etc. With an item in hand it is easy to forget the pyramid of tech and commerce that it rests on.

I think replicating old items will depend on how much of the tech, supply lines etc exist and can be restarted. Much of that stuff is only practical for assembly line modern type production & in fact gun manufacture pioneered the use of those techniques.

If each of your guns is going to be handmade by craftsmen, it might be better to start new. One benefit to Civil War type rifles is that the soldiers could make their own ammo. Those guns get the job done too.

ADDENDUM: If I were a post apocalyptic gun craftsman, the hardest thing to make from scratch would be a steel barrel that would not explode. I could imagine finding a trove of precrash steel pipes or tubes which would serve. I would make the gun around these with calibers to match.

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    $\begingroup$ Yes, you answered before I finished typing. This scenario has happened many times, Afghanistan around the English wars springs to mind. They studied European guns but couldn't replicate them with their tech, so made their own to match their needs. This was much simpler guns than we have these days. Africa they still do it. And zip guns are pretty well known homemadeguns.wordpress.com/page/2 $\endgroup$ – Kilisi Apr 28 '17 at 11:39
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    $\begingroup$ Actually, the hardest thing to make is brass cartridges. You have to make them by the thousands, and they have to fit all the guns, so standardization and mass production of cartridges is the first step. Muzzle-loaders can use lead balls (minie or otherwise) with ball molds tailored to each weapon, but even bolt-action breech-loaders need standardized cartridges. $\endgroup$ – WhatRoughBeast Apr 28 '17 at 22:33
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There is one question you haven't considered:

Why was that calibre chosen in the first place?

The calibre could of be chosen because of a multitude of reasons, which (most of the time) where practical, and would very likely still make sense in your post apocalyptic setting.

So as it would have been chosen for a good reason when it was first introduced, why waste time trying to decide what calibre to use? A lot of research would of gone into the choice, by competent people (professionals). It is possible the side looking into this could be short on time (under attack?), so this is one area they can save on.

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  • $\begingroup$ I like all except the last sentence here. The rest is good. But every time I hear "They are professionals, why doubt them?" I cringe. I work around a lot of professionals making lots of dumb choices. That doesn't make them stupid people; they are smart, and good stuff gets done too. That doesn't mean you shouldn't doubt them. Take medicine for example; many good medical doctors prefer patients who ask questions and decide for themselves if the doctor's advice should be followed. I'm in an engineering field, not medical; the people here need to be questioned a lot or bad stuff happens. $\endgroup$ – Loduwijk May 3 '17 at 15:34
  • $\begingroup$ @Aaron my reasoning behind that sentence was that "a lot of research would of gone into choosing the calibre by competent people (professionals), who you would think would know what they where doing, so why take the time to do the same work again, when you would probably reach the same conclusion again 99% of the time. But I will modify my answer to try and clarify it. $\endgroup$ – CalvT May 3 '17 at 15:42
  • $\begingroup$ Yes, that part I get, and most of that is what the rest of your answer stated. Also, the whole "just because they are professionals doesn't mean they always know best" is a personal, but very strong, opinion of mine. I know many people disagree with me. Ex: When I complain about laws that get passed, I have a relative that says "They are professional politicians, so they know better than you." Anyway, I was just voicing my opinion about the "appeal to authority." I'm very cynical, in case you haven't noticed. You're answer makes a good point anyway, so I'll +1. $\endgroup$ – Loduwijk May 3 '17 at 16:13
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It depends on how much is left over...

If some quantity of existing shells or shell casings are found, or a stash of rifled barrels are found, or a machine shop with the equipment for making a specific size barrel, or reloading equipment for a specific size round... then your first few batches will be done using that existing stash of parts/material/tools. Once those are in common(ish) usage, there's going to be a kind of institutional inertia towards changing to any other caliber or design, unless a different caliber offers significant, observable, advantages for some reason (cost, accuracy, safety, speed of manufacture).

After all, our railroads and to some extent road lanes are set at a width originally defined by Roman roads... Once a standard exists, it takes a great deal of effort to switch to something new. AKA "People are lazy."

However, if all they have are drawings or concepts of what guns once were (no salvageable equipment of any real quantity), then whatever they come up with first will last until something better is designed.

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It all depends on your manufacturing base. The entire concept of calibers is fairly new, it could only be a thing once craftsmen and machinists could reliably create multiple copies of something within specific tolerances. Matching ammunition with rifles, with as few caliber differences as possible, is sound logistics, but you need the manufacturing base to support it. Older muzzle loading muskets (smooth barrel) deliberately had loose tolerances for this very reason (and to speed up loading), the more precise rifles (had grooves in the barrel to spin the bullet) had tighter tolerances on barrels and ammunition and were much more expensive and harder to produce.

The current calibers in use today are largely based on politics. They were developed by private individuals and then adopted by governments, but there is no real reason why we have 5.56 instead of 5.75 or 5.45, or the older 7.62/.30 cal. There are a plethora of calibers out there, most used in very niche applications. The need to standardize logistics is why we have just a few calibers in military use today amongst NATO allies and the like.

But if your society is mostly scavenging tech, then it stands to reason they will repurpose current stuff as much as possible. The manufacturing tolerances are already known. You don't have to experiment as much. When dealing with explosives like firearm ammunition, you REALLY don't want to be the first guy to try something out! So current chamber dimensions, barrel thicknesses, twist rates, cartridge thickness, powder weight, and bullet weight is all well documented and established, so it only makes sense to rebuild that stuff.

But it is important to realize that the bullet and the cartridge are somewhat independent. You can use the same bullet in many different cartridges (mostly to vary the amount of powder or to get a specific shape to the cartridge. The .38 special and .357 magnum, despite their different caliber names, actually use the same bullet, the latter is just a longer cartridge to get more oomph). You can even use the same cartridge for different bullets. So depending on what materials and plans they have on hand, you may see some drift in what your apocalyptic society comes up with versus what we have right now.

Additionally the actual use of the weapon will drive caliber selection. If your guys are just hunting humans with single shot rifles, larger calibers (.30 cal and up) are usually preferred so you have good one hit stopping power. But automatic weapons kick too much with larger calibers so you see the smaller .22 (5.56mm) stuff for both controllability and magazine capacity, with the assumption that a target could get hit multiple times if needed.

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There's a somewhat exaggerated, but still essentially true, story out there about why the solid rocket boosters on the space shuttle were the size they were.

The story is that the diameter of the rocket boosters was selected because they needed to be able to reach Cape Canaveral from the Thiokol factories in Utah by rail, and the rail route had to go through tunnels, so the diameter of the rockets was determined by the size of the tunnels.

The size of the tunnels was determined by the gauge of the rails that had to go through them. And the gauge of the rails was determined by the gauge of pre-rail tramways which were built by the same people. And the gauge of the tramways was decided by matching the gauge of pre-modern cart wheels. And that gauge came from the wheels having to fit into the ruts in the road.

And the ruts in the road were originally made by Roman chariots, which were built to fit between two horses' backsides.

The thrust of the idea is that humans tend to copy what worked before, and quite often this results in odd sizes and measures being preserved when there's no technical need for them any more.

Whether this would be the case for ammunition being manufactured After the End when guns are being rediscovered is kind of a toss-up. It will depend on a lot of factors, like whether people are trying to rebuild guns from scratch or to reuse old parts, whether they fully understand the reasons behind some of the design choices made in earlier products, whether they're working within an existing infrastructure or making it all up themselves, etc.

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