A MkI Gatling Gun

If it'll fit inside your time machine, a Gatling Gun and as much ammo as you can fit. The crank version invented in 1861, you may remember its effect against pre-gunpowder armies from "The Last Samura." Provided your Hero knows what fulminate of mercury is. Now I know what you're all thinking, "That's almost impossible for anyone to make! You'd need jewelers to make the cartridges and assembly!" My answer is, "why yes, yes you would." Your hero's problem is any gun less technologically advanced than a Springfield rifle is going to be less effective than a bow and arrow. There's a reason firearms were around for hundreds of years and considered gimmick weapons as much as anything else. You'd need to turn them out in their thousands and have ammunition by the ton before your army would be superior to what it was before you turned up! That kind of investment is simply impossible for most ancient civilizations. Too many people not farming or herding or doing any of the other things you need to do to keep people alive.

A Gatling gun, on the other hand, is a comparatively minor investment in resources. Couple dozen blacksmiths and jewelers for the fine work of springs and cartridges and everything else is relatively simple. A battery of 6 divided into 2-gun sections can cover about 2280 meters of battlefield (effective range of 918 meters, leave room for overlapping fire). A pike phalanx of the Macedonian variety has about 1 square meter of space per soldier before they tighten up still further at close range. Other mass-infantry formations tend to stick to 1 square meter for each man. So (and forgive my math, I'm a history major after all!) each section can cover a frontage of about 2280 block-infantry, which'll likely be at least 8 men deep, and maybe as many as 16. In other words, a section can fire at 20,000ish men in formations so dense you almost can't miss. Probably less than 20,000 as individual units need space to maneuver, but the point is you'd need relatively few to cover an entire ancient battlefield. Enemy infantry is effectively doomed. Too lightly armored and they're torn to shreds at maximum range, and the more armor you pile on the slower they go, and the fewer of them your enemy can field against you.

Against Mass Cavalry the odds become tougher. Cavalry charged (and I'm using Napoleonic numbers here because we have the tactical manuals which give precise measurements. I assume an armored cataphract would be a bit slower, but this is a good top-end speed) at about 20kph, so 5.5 meters per second or thereabouts. That's a little less than 3 minutes to go from "maximum range" to "stabbing your gunners." A single Gatling can pump out 200 rounds per minute, so lets say 500 rounds before the cav get yah. Maybe less, since your gunners are unlikely to stand to their pieces when the cavalry is close! But even so, that's enough weight of fire to break up all but the most determined cavalry.

The other big PRO for gatling guns is that, because they're relatively large and you need few of them, is that your enemies are MUCH less likely to figure them out. Easy for a foe to steal a musket, or buy one from a shady supply soldier or get one from a deserter. But Gatling guns are much harder to just walk off with. Plus they become "prestige" weapons for your king/emperor/Hero, which is always a plus.

You also get the psychological impact of literally mowing down ranks of the enemy in ridiculously short amounts of time. in 5 minutes your 6-gun battery will have fired about 12,000 rounds at the enemy. Even if only a quarter of your rounds hit (likely, maybe even too high, because of a whole host of factors I won't get into) that's hundreds or thousands of the enemy dead and wounded. Many at a range they weren't even expecting danger. Despite the bloody toll of many ancient battles, actual COMBAT deaths (as opposed to massacred-while-running-away deaths) were generally few. If you kill a couple thousand enemy while they're still 450 meters away (outside maximum bowshot) the whole army may well break under the pressure, leaving your forces to mop up. Best bring a good amount of light cavalry with you!

A small but important upside to a Gatling Gun is that, once you know how to do that, you can also make things like siege Cannon with a little bit of experimentation. Or standard muskets/rifles, but as I've mentioned before I think those aren't really useful in societies where 95-98% of the population has to farm or else there's a famine.

The downside, apart from your hero needing to lug your demo piece to the first king he comes across, is that they eat up ammo at a ferocious rate. Which in turn means you still need a lot of materials. For instance 40 cartridges (which you'd 100% want to recover after a battle) take a pound of brass. If you have 12 guns and each one carries just 5 minutes of ammo, that's 300lbs of brass alone. A lot, but still MUCH less than the material cost you'd incur for equipping 1,000 men with flintlocks. Which again, wouldn't be as useful as 1,000 men with any number of different types of bow/crossbow.

Of course, what would likely end up winning your hero the war wouldn't be the firearm he brought back, but his insistence on basic sanitation. But teaching a civilization to wash their hands and piss downstream is less fun than superior firepower!

Why is he bringing a gun to duplicate? He should be bringing books! The gun would only be to demonstrate what the objective is, it need not be copied.

The problem with reproducing modern firearms in pre-industrial civilizations would be:

• steel of inadequate quality
• lack of precision tools
• lack of an industry capable of manufacturing smokeless gunpowder.
• severely limited possibilities for economics of scale.

The cause why people in the middle ages didn't make cartridge-based automatic firearms wasn't because the idea didn't occur to them. The reason why many inventions weren't "discovered" much sooner was that they lacked the economical and industrial base to manufacture them efficiently and economically.

Actually, some repeating firearm designs already emerged around the 16th-17th century, but they were so impractical that repeaters didn't get useful and widespread until the late 19th century. Those early designs were so expensive to make and so difficult to maintain, that they never saw widespread use.

Even if, with a lot of handwaving and a lot of luck you managed to replicate a modern firearm in a pre-industrial age, it would be of questionable quality and reliability, and as cartridges have to be manufactured at an extremely high level of precision, you would basically require the most experienced master jeweler to manufacture each and every cartridge by hand, possibly requiring most of a day just to make a single one. And then you would have a "modern" firearm of extremely poor reliability with a high likelihood to jam, break, or explode in your face after every shot, with ammunition so expensive that for the cost to make one full magazine you could hire a whole army of mercenaries.

This is why those early repeaters, made many centuries before repeating firearms became commonplace, didn't have cartridges. They had a reservoir of loose gunpowder and loose lead balls, and an intricate clockwork mechanism driven by a had-crank which performed the motions of loading a traditional muzzle-loader. Yes, they had, for their time, an amazing and unparalleled rate of fire! Still, they were too expensive and too difficult to maintain to see any widespread use. Every single part needed to be made individually, and you couldn't use spare parts from one gun to repair another one. An army requires weapons they can maintain and repair in the field, you can't drag your kingdom's most experienced master smith and his whole workshop with you wherever you go. And even if you do, he alone couldn't maintain the guns of a whole army of tens of thousands. This was one of the reasons why the Girandoli air rifle failed so miserably. I recommend to watch the linked video, because it explains well why extremely powerful but expensive and complicated weapon systems often fail against less powerful but cheaper mass produced ones. For a complex piece of equipment you need a complex infrastructure to keep it in working order. You can't just put it into a soldier's hands and hope you've done all the work.

So, the highest level of weapons technology you could ever hope to replicate (and use in an effective and efficient way) in pre-industrial times, would be muskets with Minié balls, and cap-and-ball revolvers. They became obsolete 150 years ago, and even they would be just on the very verge of possibility. Anything newer, you can forget about it.

Making firearms may not work out but just go back further. The Greek shields and 20' spears allowed them to dominate the world. A Roman general said getting through 3 layers of spear points to be able to start fighting Greeks was the hardest battles he fought. Greek fire won naval battles. The Romans interlocking shields won for them. The Indian's using elephants as a shooting platform dominated war in its sphere. The Chinese cross bow allowed one Chinese emperer to overcome all his rivals and unite China. The Mongul's horses dominated the plains. Just changing some tactics and fighting skills allowed Shaka Zulu to dominate southern Africa. The first chariots gave an incredible advantage. Catapaults and trebuckets changed war throughout europe and the known civilized world. Hot air baloons made observation and communications leap forward. The various armor changed and evolved. The long bow had it's own influence. Compound bows would allow distance and accuracy seldom available throughout history.

If you go back before these inventions, you can apply all of them at once and be virtually invincible. Now you just need to learn ancient Sumarian. Going back before written language would again be a HUGE advantage to the first warlord to use it for communications.

I'm going to do two frame challenges: firearms are not the best way to do this and the hero can't achieve his goal.

First of all, as others have stated, modern firearms are made with modern tools and modern materials, which simply won't be available to the hero back whenever he wants to go (presuming he wants to go back when modern firearms would have a big enough impact). Non-modern firearms or knowledge about them can indeed be useful, and the hero could very conceivably introduce gunpowder and cannons to the early middle ages, rendering most (if not all) castles of that age VERY vulnerable to your army. The problem here is that your army so far consists of the hero and his cannon.

You see, your hero is still just a peasant, even if he possesses some magical stick that can kill at a distance even through armor, or some cart that can hurt a castle. His best bet is to hire some blacksmith to build the parts, produce the gunpowder himself and sell the weapons, which can turn him into a very wealthy merchant very quickly, but you're putting these advanced weapons in the hands of anyone who can pay for them (and who will eventually oppose you when you want to conquer them along with the whole world). Still, we might be on to something.

Since your hero needs to hire/get his own army (arming a noble's servants is actually a good deal, but doesn't make the hero the ruler of the world), he'll need either noble blood (good luck with that), a huge wealth or a noble title (acquired, not inherited). The last two can actually be achieved in a similar fashion: bring back something other than firearms, some knowledge or technique (or even better, multiple ones) that allow you to mass produce or somehow acquire something of value which he can sell for huge wealth, or that he can sell the secret itself in exchange for nobility and vassals. A wide array of techniques and knowledge works best, for example faster, ocean-faring ships to support your trading empire (plus they will help a lot later on with your conquest), irrigation techniques (you will eventually become a ruler, and you need to keep your empire fed), and just a general understanding of logistics, law, ethics, anything that helps your empire last until you conquer the whole world. Which brings us to our final problem (and second frame challenge).

Your hero can't conquer the whole world in his lifetime. The world is simply too big to conquer with the general level of technology that your hero needs to encounter to make modern firearms game-changing enough to even give him a shot at conquest. I would even argue that the world is too big to conquer with any pre-nuclear technology.

One option would be to reduce the size of the world. Build a bunker, detonate enough nuclear warheads to kill all life on Earth, and now "the world" is your bunker, where your hero can rule until his probably very early death.

Firearms require a social and industrial ecosystem to exist. Modern weapons with metal cartridges could not exist before the development of deep draw dies and the skilled workers to use them. Even as late as the 1860's (American Civil War) cartridges were paper cylinders holding the powder and the ball, but now you need a cheap source of paper (and preferably waxed paper to protect the contents).

This process is "rinse and repeat" in any age, gunpowder was described as early as the 9th century AD and the very first primitive firearms appeared shortly thereafter. Improvements in the formulation of gunpowder required improvements in metallurgy and manufacturing to take advantage of them (the etymology of "barrel" is exactly what you think: the early gun barrels were made is the same manner as wooden barrels...).

The first firearms were made at a time when metallurgy was not advanced enough to cast tubes capable of withstanding the explosive forces of early cannons, so the pipe (often built from staves of metal) needed to be braced periodically along its length for reinforcement, producing an appearance somewhat reminiscent of storage barrels being stacked together, hence the English name.1

For all practical purposes, an arquebus or other matchlock weapon is probably within the skillsets of trained workers as far back as the late 1300's or early 1400's

Hand Gonne in the 1300's

Matchlock firearm

However, even jumping forward a few centuries with a matchlock isn't going to do the mad scientist much good. The production of gunpowder and firearms is essentially the domain of a few skilled craftsmen, the economy isn't going to support the large scale production of firearms and the tactics needed to effectively use firearms also needs to be developed, along with the logistical infrastructure to support firearms (imagine transporting tons of gunpowder across Europe in the 1300's...). Indeed, these sorts of firearms require a specific set of tactics (sometimes known as "Pike and shot") to allow the gunners to be effective against mounted cavalry.

Pike and shot army

So in isolation, the knowledge of advanced firearms alone isn't going to do much good. There must be an economy with enough capacity to produce the firearms, gunpowder, skilled craftsmen, transport (carts, horses, special containers to limit fire and explosion risks), sergeants and officers to develop and train troops in the right tactics...Your mad scientist will need to bring an immense illustrated library and probably an entire team of expert instructors in many different fields in order to make real changes to history.

I'm going to disagree with some of the naysayers and point out that as soon as gunpowder is discovered, they have all the technology they need to make any manual action gun. Revolvers, bolt action, lever action, break action or pump action long guns are all possible.

Some posters have pointed out that pre-modern metallurgy is not up to our standards. I agree. But you don't need it to be. You can simply use thicker chambers, and remember, black powder produces much less pressure than modern smokeless powder.

Some have pointed out that pre-modern machining is nowhere near as good as modern machining. Again, I agree. And again, with manual action guns you don't absolutely need it to be. Semiautomatic or automatic guns need machined precise to the micron because they work together. Every part has to work together precisely or the whole thing won't work. Manual action guns on the other hand have much looser tolerances. Tighter tolerances are still nice, but loose ones are not dealbreakers.

The key is to not just bring back a gun or blueprints for a gun, but also the recipe for fulminated mercury so you can make cartridges.

Bring the Tools, not the Weapons

The hard part here is getting the precision tooling you will need done right. Rifling a barrel and pressing ammunition is pretty darn hard without the right equipment, but all the tools needed to set up a fire-arms and munitions workshop take up surprisingly little space.

Since he has enough wealth in the 21st century to acquire significant amounts of gold and build a time-machine, I'm assuming he's fairly wealthy. Although he lacks mechanical engineering skills himself, he could prepare for his mission by hiring proper engineers to modify hobbyist gun making kits to rely on non-electrical power sources such as foot-pedals, and help him design weapons to the specifications of the alloys that will be available to him in the past.

Before he goes back, he should then run tests making his own home-made bronze, gun powder, ignition caps, etc. to test his ability to make sound weapons from what will be local resources.

The Time & Place

For this I'd suggest going to Ancient Athens at ~600 BCE.

While most historical guns were made of cast iron or steel, bronze is actually a fairly good material for making firearms. A revolutionary era bronze cannon was half the weight of the cast iron cannons of the time. Steel did not really get good enough to compete with the quality of bronze as a gunmaking material until the 1800s, so your time traveler should head back to the bronze age before gunpowder was first invented. This means that in the bronze age you will have a good material to work with and be able to control the means of producing and distributing the ammo so that even if another nation reverse engineers your gun, they won't have the means to make the ammo.

Another reason is the culture. Most bronze age civilizations were ruled by nobility such that you could never gain a position of power among the people. No matter how good your tech is, you would never be more than the king's slave. In Ancient Greece, money could buy political power. You could raise your self up to be a general, and use the influence of your conquests to eventually usurp or at least strongly control the democracy.

Athens also has a relatively large population in a small place with lots of allied city states. This gives you the manpower you need to expand far.

They also have the trade routes and craftsmen you need to acquire and manipulate all of your materials.

The Currency

You may want to exchange most of your gold for silver before you go. At today's market value, gold has an 80:1 exchange rate with silver, but in ancient Greece, the exchange rate would be anywhere from about 13:1 to 1:3 just depending. Silver (and not gold) was the standard currency in ancient Athens and the preffered coinage for international trade on the mediterranean (4.3g was worth 1 day's labor). Greeks specifically did not like being payed in gold due to efforts by Persia to devalue thier silver coinage by intentionally fixing the exchange rate of gold and silver to prefer thier coinage at the 13:1 ratio, even though gold was considered less valuable than silver in Greece. So, definitely bring some gold for any materials you need to buy from Persian merchants, but silver is the way to go for local labor and materials.

The Weapon he Makes

To dominate the ancient world, your weapon needs a few qualities that not all historical firearms could achieve. First, it will need to be able to kill through the shields and armor of the time period you go back to. This significantly limits the effectiveness of shotguns and low calibre firearms since available resources will not be able to achieve the muzzle velocity and armor penetration of modern munitions. Secondly, it will need range. To have a true tactical advantage, you will need to be able to shoot down enemy archers, slingers, and crossbowmen before they can start shooting at you. Most muskets are only target accurate at 100m and area accurate up to 300m (similar to ancient bows), but even primitive rifles could get you target accuracy in the 700-1800m range. Third, you will need a enough rate of fire that your armies will get in many shots before your enemies can close range enough to take away your advantages. Automatic firing systems might be too complicated to reproduce since you won't be able to produce high quality springs, but revolvers should be able to be produced without them.

Other considerations are that manual bullet press does not need to be big, and can make thousands of rounds of ammo a day; so, you can skip the ramrods and go straight to self contained munitions for ease of reloading. The hard part is going to be blasting caps since they require chemical processes that would be hard to replicate in the ancient world without an extensive knowledge of chemistry... but I'm sure your time traveling inventor could figure it out.

The final weapon will likely resemble the Whitworth Rifle as suggests UIDAlexD in terms of accuracy, calbre, and muzzle velocity. However, the addition of easy to add later tech like cartridge bullets and a revolver cylinder make it a much better weapon.

Springfield Model 1861

As long as you go to a place where people have no problem creating iron swords and equipment you should be able to get them to create a (initially probably unrifled) version of this gun that works with a minimum of tools.

At first your engineers (let's assume you go to India at 100 BCE and start your conquest from there https://en.wikipedia.org/wiki/History_of_metallurgy_in_the_Indian_subcontinent#Iron and elephants hate being shot at by guns) will probably turn out something more akin to the Brown Bess rifle https://en.wikipedia.org/wiki/Brown_Bess. But this is fine! You are taking the Springfield muzzle loader because it was one of the last muzzle loaders in use, so it contains all the useful features that you'd like to eventually be implemented, but even a poor copy will be very functional.

I highly recommend using minié balls as soon as possible for ammunition. These are cheap to produce and very powerful, and will have no problem punching through any shields (think the shields of the Roman empire) or horses (think Parthian horsemen). The rate of fire will be approximately 3 RPM, which means that archers will actually have a rate of fire advantage over you, and might even have an accuracy advantage too while you are still using unrifled muskets. However this brings me to the most important advantage of using muskets over bows:

"No" training required

While you are still using the brown bess version, an individual musketman would probably be at a disadvantage to say, a trained Parthian horse archer. However, training a soldier to use a musket takes weeks, training a soldier to wield a bow takes years, not to mention the costs associated with horses and the logistics required to field them.

Your greatest advantage will therefore be the size of the armies you can field. With massed musket fire, no enemy troops will be able to get close to you. Nonetheless, issuing a detachable bayonet to every troop will be highly recommended since it allows them to hold the line while the second (and optionally third) lines keep up their withering fire.

To sustain such large armies, you'll need to bring 2 additional items:

• A handbook on medicine and hygiene: before the 20th century (and even during it), most casualties in armies were caused by disease. Without hygiene, and proper training of your officers about where to setup camp and how to guarantee a proper water supply, your army might quickly have plenty of guns, but not enough people to use them!
• A book on constructing a field kitchen: The field kitchen allowed armies to cook the meals for soldiers, thereby severely reducing the amount of time spend by soldiers on foraging and cooking their food, and therefore massively increasing how far your army can move in a day. You will need to move quickly to giving other empires the time to copy your firearm design and field their own versions.

If your time traveler is trying to alter history by introducing "modern" firearms long before their time, his best bet would be an external hammer break-action shotgun (single or double), with brass cartridges loaded with black powder. The cartridges should be a mix of birdshot, buckshot, and "pumpkin ball" slug loads. Fixed chokes, no tighter than "improved modified", recommended. That's enough to demonstrate the effect of choke on a shot column (especially if one barrel is cylinder and the other improved modified), but not so tight as to cause trouble with the round ball loads.

If he's got any chemistry knowledge at all, he can make primers that will work, brass was a known and worked metal even in ancient times, and black powder shotguns can be made with pretty primitive steel (or even bronze) for the barrel and lockwork.

With a source of cartridges, a competent smith from 1600 (possibly as far back as 1500) could build a shotgun that could fire modern cartridges. It wouldn't be a nice to fire as a modern one (heavier, for one thing), but it would work. Once they have working firearms, the advancement can begin -- they'll be effectively up to 1860 technology as soon as they can make cartridges and that gun.

He should bring back a replica firearm from a historical period as close as possible to the one he's trying to dominate. The precision manufacturing and metallurgy and chemistry of the time will fundamentally limit reproduction, so picking something that could already nearly be made will help tremendously.

Note that he still has his work cut out for him. Despite outward appearances, guns don't always mean you win. It takes training. In particular, training entire militaries is a very time consuming task that may require multiple lifetimes to achieve.

For a cinemagraphic depiction of this, consider the first part of The Last Samaurai. Fighting against trained warriors when you are unfamiliar with the tactics regarding how to use a weapon, nor the strategies which let you leverage it, can go spectacularly wrong.

Unless he's an expert metallurgist the biggest issue you're going to have isn't in terms of producing the parts; anyone with a drill and a hand file (or some rocks that will serve the purpose) and enough patience can make a gun from a solid block of appropriate metal and therein lies the problem. Modern weapons require modern alloys, many of which are proprietary products that the character cannot know the recipe for. Furthermore the fineness of measurement necessary to make them if the recipe is known has only existed for a few decades, some of these alloys require measurements in the parts per million.

All is not lost however, you can take some weapons that are currently antiques back a lot farther than you'd think. Assuming your character knows how to make reliable gunpowder that is. You could take a Culverin back to 3300BC Anatolia and be able to reproduce it in local bronze.

Nothing.

No modern firearm or ammunition will pass for an ancient one. Every speck of metal and every chemical trace will make it stand out as not belonging. Worse no one is reproducing modern firearms or ammunition prior to the ~1800's and metal lathes, chemistry, and precision measurement.

Basically if they can make a firearm, they are already making said firearms. If you want to advance technology you need to come in with the specialized knowledge involved you can't just hand them something and say make this, because the answer will be "how?" especially if chemistry is involved and it almost always is.