The idea of the democracy.
Ha-ha-ha, I only joked. Normally, I don't post popular bullshits with the intention to collect many upvotes, but it was so obvious that I couldn't miss it.
The democracy was well-known already since the ancient greeks, any educated people in the medieval age knew that it existed, and that it would be possible also in their era, and they rejected it. If a strange alien - arrived probably from far, far away, from a terra incognita (this is what they would think from a time traveler) would propagate democracy for them, they had laughed on him. They would say around so: "look these primitive peasants, they can't even read, they can count only until 10, except whose fingers were already cut! And you want them to select our King? Look, what Socrates wrote from their "democracy", it was actually the rule of the idiots!" So, the idea of the democracy would be totally ineffective, it is nearly sure.
So, don't have illusions, a weapon means a weapon and not an idea.
It should be a primitive weapon. You can't get even an AK-47 (alias Kalashnikov) with you, because they don't have the required metallurgic industry to reproduce and service it.
Here comes the next problem: any machine - also the weapons are machines - were produced by other machines. With your pure hands, you can't produce anything. Maybe with a lot of weaponsmithy work, you can produce a steel tube (what will be the barrel of the AK-47), but it won't be enough precise. The bullet won't be able to leave it.
On the end of the chain on machines produced by other machines, there are always human hands. Human hands, shoveling coal into the steel forge, controlling the machines casting the iron, and so on.
What they didn't know:
- They didn't know the exact chain, how to produce finally AK-47 from the people having no machines only hands.
- They didn't knew the explosives.
But they had a lot of smart people. Consider Newton, Galilei - millions and millions people had existed with their mental skills also in their era, but they never became well known. Most of them died as a peasant, and they were known only in their village, because they were the other (after their priest) who could read.
So, take a lot of good books with you, with university text books about metallurgy, chemistry.
Take some explosives, too, and an AK-47.
You will be a chief scientific advisor of a local power, maybe even a king. In some decades, you will be able to produce an AK-47.
It is very important to get enough supply to show a local potentat, that you can produce unimaginable weapons. There is no such stupid power in the era who couldn't realize its importance. It is the result of a selection mechanism: anybody decision maker underestimating the worth of the weapons, had been long conquered by others who hadn't.