Eureka! Our hero just discovered the secret of time travel, but what to do with it? "I know!", he says. "I will travel back in time as far as I can and hand them a modern firearm. They will learn how it works, and they will be technologically superior and dominate the world with me as their leader!"
At least, that's the idea.
The Time Machine
The Time Machine is a rough prototype, so it is best not to travel back and forth too often. In fact, once jumped back in time enough, it's possible that you can't ever jump back forward. As a result, the hero cannot just continuously jump back and forth in time to transport a lot of modern equipment. He has one chance, and one chance only.
Furthermore, the Time Machine is able to travel through time and, contrary to the name, space. The Time Machine can pick any past date, time and location, and the Time Machine would bring the hero there. It is not perfectly accurate though, so you might end a year sooner or later, or somewhere within a kilometer of the target.
Our Hero is a scientist, but not a brilliant engineer. While he does have rudimentary knowledge of firearms, he cannot perfectly describe what every single component of the gun does, nor how it is produced. He might be able to explain that a striker works by using a spring to propel a metal stick against the primer, which explodes, in turn ignites the powder and thus propels the bullet forward.
He is also not a chemist. He doesn't know how modern smokeless powder is made, but he did look up how old black powder was made and he is reasonably certain he could explain the process, if necessary.
Speaking of explanations and speaking about things, our Hero is also a gifted linguist and able to speak any language, old and new, to a good enough degree that he could express even more technical concepts to native speakers.
Our Hero wants to bring any modern firearm - a pistol, an automatic rifle, a shotgun, etc. - as well as a reasonable amount of appropriate ammo - with him into the past. Our Hero wagers that he would have the biggest chance of success of dominating the world if he would travel in time further back. As such, our Hero wants to pick a gun that even a rather old - by whatever comparison - civilization could reproduce in a good enough quality.
One can assume that the Hero is patient, so if the good people of that time require a year or two to understand the firearms, that is acceptable too, as long as it results in them being able to make the firearm afterwards.
The required time is also not very important. Equipping an army takes time, and our hero knows that. So if arming an entire army with firearms takes several months or even years, so be it.
We can further assume that our Hero brings enough Gold with him - a currency that is universally accepted anywhere - should the need to pay someone arise.
- Which firearm should our Hero bring with him, and why?
- How far back in time could he go, before even the simplest modern firearm would be so technologically outlandish, that not even the most skilled scientists and engineers of the time would be able to reproduce it?