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In my world a a several members of the US Armed Forces get transported to a medieval world. They have only a limited amount of resources and they must find a way to replenish their supplies. Specifically, after some conflict with the natives, they are in desperate need of more bullets they didn't any tools from our world with them but one of them does remember formula for making gunpowder.

Is it possible for them to construct more bullets so that they don't run out?

they only have 6 months before they get attacked again once that happens their run out.

They also have help from some villagers that they have helped.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – James Jun 20 at 20:57
  • $\begingroup$ For an interesting discussion of this problem, read Harry Turtledove's The Guns of the South. It is an alternate history in which time travelers deliver AK-47 guns and ammunition to the Confederate armies in the US Civil War. General Lee gets interested in reducing dependence on the time travelers, and has discussions about replacing AK-47 ammunition using only 1860's technology. $\endgroup$ – Patricia Shanahan Oct 10 at 15:27
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They can easily restock in 6 months, assuming one of them knows a bit of chemistry (and get a bit lucky), but they will have fewer and fewer replacement bullets after each attack.

They are limited to the number of usable shell casings they can find; they will not be making anymore casings; the technology just isn't available for them to make the right kind of metal or doing the precise enough extrusion. Making anything close to cartridge quality brass before the 18th century was all but impossible since they could not refine zinc, ditto for aluminum and machinable steel. There was a question about this before, Could medieval people produce automatic firearms if they had access to the schematics?

Casting the projectile is easy, they can literally pull one out of an unfired round and make a clay mold from it. They will have to polish the rounds before using them but that's just time consuming and time they have.

Making gunpowder is fairly easy provided they can find a source of sulfur, which is not too difficult. They might have to do some traveling to collect enough depending on the size of the town. Saltpeter would actually collect on root cellar walls in many places and was used for preserving food. Which part of medieval you mean is an important consideration, as cannons and gunpowder were around for most of this period.

Making primer is a bit harder but thankfully primer caps can be reloaded. However, the best bet they have is using a mercury based primer, which will quickly destroy the brass case with repeated use. Potassium chlorate is possible, harder to make but easier to find materials, however it will damage the barrel of the gun over time. Making primer will easily be the most time consuming part of reloading. Interestingly if they have even a single phosphorus grenade, they can use it for primer for several dozen, if not several hundred rounds, and it will not have either of these problems.

Their biggest problem is that guns don't use gunpowder anymore. Guns use higher power mixtures that produce much less residue. The newly made bullets will quickly foul mechanisms and barrels, and may not be powerful enough to cycle weapons. They will be reduced to single shot and manually cycling their weapons with the new bullets as they will be unreliable in automatic weapon.

They might even be able to make some nitrocellulose if their working knowledge of chemistry is good enough, that will solve many of their problems with power and fouling, it is mostly about refining some nitric acid. This does require quite a bit more chemistry knowledge, but not an unbelievable level, especially if they have a demolition expert.

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    $\begingroup$ An alternative use of such chemistry skills would be to make hand grenades, if they get a few friendly locals with pikes they can wreak havok with enemy infantry formations and most likelly break any cavalery charge $\endgroup$ – lijat Oct 4 '17 at 8:24
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    $\begingroup$ Do the casings need to be made of high quality brass/steel? Would the bullets work if a lower-grade steel was used? I also think people tend to underestimate just how good at making steel medieval people were. $\endgroup$ – Korthalion Oct 4 '17 at 10:35
  • $\begingroup$ no, not in modern firearms, too much risk of them failing and ruining the gun itself and possibly injuring the soldier. Without quality metal even shaping them in to casings would be all but impossible. $\endgroup$ – John Oct 4 '17 at 13:38
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    $\begingroup$ @Korthalion Cases need to contain tens of thousands of PSI of pressure and must be shaped to perfectly fit the chamber of the firearm. The industrial processes necessary to produce brass or steel cases that could stand up to smokeless powder didn't exist until the late 1800s- it's not about the quality of metal, it's shaping it consistently. $\endgroup$ – Catgut Oct 5 '17 at 18:49
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    $\begingroup$ @elemtilas casings are not infinitely reusable, everytime you reuse them you loose material because they have to be trimmed to fit properly. $\endgroup$ – John Jun 21 at 1:07
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Actually, casting bullets from lead isn't difficult at all (simply extract a bullet from its' cartridge and use it to make a mould to cast fresh ones out of lead).

The tricky parts are recovering the brass casings, and making fresh primers.

The brass casing is an essential part of modern firearms. It mechanically holds all the parts together (bullet, powder or propellant and primer) in the proper mechanical alignment to work with all the other mechanisms of the firearm. It provides protection from the elements and temperature extremes while the rounds are being transported, and upon firing, the brass is flexible enough to expand slightly to seal the chamber (a modern rifle can produce 50,000 lbs/inch^2 of chamber pressure), then contract as the pressure drops, and carries a lot of the heat from the chamber on extraction and ejection.

Drawing the brass with enough precision to make an acceptable case might be possible with medieval technology, although I would have doubts. The link above shows the process in some detail, which suggests that the high precision cases needed by modern weapons are not going to happen, instead each case must be painstakingly hand crafted to ensure a proper fit.

One other issue you haven't really addressed is black powder isn't very effective as a propellant compared to modern formulations, and is dirty and corrosive to firearms. Weapons and even automatic weapons have been made to use black powder. Early Maxim guns used black powder cartridges, and earlier "cranked" weapons like Gatling and Nordenfelt guns also had been made for black powder, but remember these were designed to use black powder and had greater operating clearances and highly trained gun crews who stripped and cleaned the weapons on a regular (daily) basis under the supervision of the Sergeant. M-16 rifles during the Viet Nam war were plagued by problems because the solders were issued ammunition with a slower burning "powder" which fouled the weapon, and were (amazingly enough) told they did not have to clean the weapon. This problem ended with ruthless cleaning and inspections by the unit NCO's.

enter image description here

Three barrel Nordenfelt gun on a field carriage

Of course, there must be a reason to use firearms rather than adopt local weaponry. Building steel crossbows would be far easier and more efficient, since it allows the heroes to arm virtually everyone in the village with an effective weapon (capable of felling knights), and providing a large numeric edge over the highly trained and very expensive fighting men (knight and Men at Arms). This is the essence of the Infantry Revolution of the 1400-1500's, where weapons and tactics were developed to allow large numbers of men with limited training to effectively contest the battlefield against smaller numbers of highly trained warriors.

enter image description here

lots of these guys make up for lack of firearms

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    $\begingroup$ Note that there's no such thing as "its' ". Your options are "it's" (short for "it is") and "its" (meaning, "belonging to it"). $\endgroup$ – David Richerby Oct 4 '17 at 9:40
  • $\begingroup$ Isn't the brass casing reusable? $\endgroup$ – Herr Derb Oct 4 '17 at 9:46
  • $\begingroup$ The casings are reusable to a certain extent, so the heroes need to recover every single one. As well, the casings need to be inspected for damage, as they could be dented, bent or even have their bases torn off by improper extortion /ejection. After a while it would be far safer to melt down the brass and make fresh casings from scratch. $\endgroup$ – Thucydides Oct 4 '17 at 20:39
  • $\begingroup$ Brass casings expand when the round goes off and may need to be 'resized' before the next use, depending on the action type of the weapon. $\endgroup$ – user2448131 Oct 4 '17 at 21:36
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They can make bullets, but as Slarty says, they cannot fire them without a percussion cap to make the gunpowder go bang. Unless they know how to do that, they cannot use their modern weaponry.

So they need to go back to basics and build themselves a matchlock. If the locals have tinderboxes with flint mechanisms, a flintlock might be possible instead. They will need to dismantle their weapons and reuse those barrels with a new breech, because those rifled barrels will be infinitely better than anything they could make themselves, and presumably carving their own woodwork for the body of the new rifle.

The most important thing they have though is knowledge of tactics. If they train up villagers, they may not be able to equip them all with rifles, but crossbows should still give them a good edge. The chances are very good that their adversaries have never seen modern infantry tactics. The Boers pioneered that kind of skirmishing, and they fought the British Army to a stalemate with much smaller numbers and much worse equipment.

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Yes. If they know how to make gunpowder, then the rest is pure mechanics. The hardest part is producing viable bullets. Lead casting is an ancient art, older than bronze, and achievable with a good hot camp fire. But other materials might work if not as well (dense hardwoods for example). After that, every gun enthusiast knows how to reload a cartridge.

A longer term consideration is wear and tear on the barrel of the firearm. It's not such a big deal with a shotgun, but it will become a concern with a rifle. Rifling a barrel is something that very few people can do well even with modern tools (my father was one such). Using ammunition that does not fit the barrel as well as mass manufactured rounds will cause more rapid wear and tear, and the barrel will not be replaceable.

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  • $\begingroup$ Using the emergency rounds, I imagine there will be more accidents when trying shots from the gun. Worst will be the gun will explode because of blockage. I'm not an expert, but I think modern gun has mechanism to prevent unsuitable bullets from being used? $\endgroup$ – Vylix Oct 4 '17 at 4:22
  • $\begingroup$ dose anyone know if those mechanism can be disabled, without damaging the gun? $\endgroup$ – Bryan McClure Oct 4 '17 at 4:25
  • $\begingroup$ A gun, modern or not, is a tube and a place to hold a projectile and an explosive charge. If you arrange a projectile and an explosive charge properly in the chamber, and can set off the explosive charge, it will fire. Everything else is pure mechanics and/or window dressing. $\endgroup$ – pojo-guy Oct 4 '17 at 4:28
  • $\begingroup$ The nearest thing to a "mechanism to prevent unsuitable bullets from being used" is that cartridges that are too large cannot be inserted into the chamber, and if they are too small the firing pin won't hit the primer. They would want to reload their cartridges, so neither of these applies. When their cartridges start to wear out and need replacing, you might start to get some unsuitable rounds until they learn to make the cartridges consistently the same dimensions. $\endgroup$ – pojo-guy Oct 4 '17 at 4:34
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    $\begingroup$ The guns will work, but ballistics will suffer from lack of pressure. Gases might corrode the internal lining of the barrels due to different chemistry of smokeless ammo versus smoke ammo. Gunpowder generates less pressure peak than smokeless. Guns will progressively increase dispersion due to wear and tear, and they cannot be replaced due to the difficulty of producing materials for this. You can replaced the rifled barrel for a unrifled one, but it will be pretty hard to build primers with the technology of the time. $\endgroup$ – Jorge Aldo Oct 4 '17 at 4:46
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The two big problems they would face would be finding sufficient raw materials and adapting modern weaponry to use a different propellant, casing and bullet.

They would need a source of lead. This might be found in church windows (although the locals would be up in arms over that) and a mould (probably doable). Much harder would be making any form of useable propellant. Assuming they were trying to make black powder they would need sulphur, potassium nitrate and charcoal.

Charcoal would be easy enough but grinding it sufficiently finely and avoiding any larger particles would not be easy. Flowers of sulphur might be available but then again might not and that would be the end of it unless they knew a lot about chemistry and had access to the right reagents (unlikely).

Potassium nitrate would pose huge difficulties. Extraction from dung (the only source for large quantities) would be a slow messy and imprecise process. The resulting material would need to be recrystallized probably several times and yields would be low due to wastage at all stages. The final product would have to be dried and ground and then mixed in accurate proportions with the other ingredients. Would they have a balance with them or rely on the locals? Would they know the exact formula?

Despite all care taken, such make shift gunpowder would not be very pure, homogeneous or dry. Once installed in the breach and loaded there would be no way of setting it off unless a hole was drilled in the back of the weapons to apply a lighted splint. Even then modern rifled weapons would quickly become fouled and require elaborate cleaning. As for automatic weapons there would be no chance of them operating effectively and they would probably present a greater hazard to the owner than the enemy.

In short it might just be possible to make a modern weapon fire a bullet in such circumstances, but it would not be an effective weapon. It would be more effective to use any powder they have available to make grenades, although fuses would not be reliable or easy to make. A better bet would be to make some sort of terror weapon. Basic black powder in a tube with a whistle can make a lot of noise which would frighten the enemy especially their horses. Better still would be to make other weapons such as cross bows as already mentioned.

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  • $\begingroup$ Lead was used for all sorts of things in those times -- roofing, plumbing, cosmetics, medications, oh, and glazing! -- easy enough to find without breaking the windows. $\endgroup$ – elemtilas Jun 20 at 21:32
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If they have an armorer and an officer with a bit of chemistry knowledge, they would be fine. Rifled barrels aren't that hard to make if they have enough supply to buy them time and training hours. They would have to recruit at first to get firepower up until they could get lathes and mills running. If they could use the lead chamber method to make sulpuric acid, they can make nitric acid from nitrated earth. That gives them a path to cordite, which, although unstable and dirtier than normal smokeless powder, can be used at a higher ROF to give them machine guns back. The logistics knowledge, the medical knowledge, etc would be enough assuming a well rounded, well trained officer contingent, armorers, and engineers. A platoon of grunts and a fresh out from college without support staff are going to have a hard go of it, no matter what.

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  • $\begingroup$ What about the primer? They'll also need to save their brass, as casings tend to have much tighter tolerances than you're likely to be able to produce in a medieval setting. A rifled barrel might be difficult, at least to NATO standards, as medieval metallurgy isn't that great. Which actually begs the question, what kind of arms? It'd be easier to keep an AK-47 working than an M-4 in OP's setting. $\endgroup$ – stix Oct 10 at 15:04

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