Minié ball.

The problems with modern weapons have been discussed several times, with all the problems of re-creating automatic firearms in a medieval setting. Basically, without industrialization everything must be hand-made, and even if they managed to make a modern firearm, with every single bullet carefully hand-made by master jewelers, it would cost so much it would not be worth it. Better equip 1000 men with slightly better than contemporary weapons than 1 man with a modern weapon.

Assuming they already have gunpowder, introducing standardized calibers, paper cartridges and the Minié ball would have a huge impact, without requiring any special technology or chemicals unknown to the era. The problem with chemicals would be, that even if you knew what it was, you could probably not locate it and start mining it, and even if you managed, you would need a large infrastructure to make use of it.

Assuming you could get into a position to explain your ideas to someone with enough power, you could make their guns much more effective with historical knowledge and very little to no new technology.

Standardizing calibers would make their guns much lighter, easier to load, and less prone to misfire, besides making logistics easier. They had rifled barrels in the early times, but they didn't use it much because it was a nightmare to load, prone to misfire, and you needed to clean your gun after almost every shot, or the residue caused the next bullet to be stuck in the barrel. With the ingenious idea of the Minié ball, you could make their gunners much more effective, by significantly increasing both their firing rate, range, and accuracy.

Minié ball.

The problems with modern weapons have been discussed several times, with all the problems of re-creating automatic firearms in a medieval setting. Basically, without industrialization everything must be hand-made, and even if they managed to make a modern firearm, with every single bullet carefully hand-made by master jewelers, it would cost so much it would not be worth it. Better equip 1000 men with slightly better than contemporary weapons than 1 man with a modern weapon.

Assuming they already have gunpowder, introducing standardized calibers, paper cartridges and the Minié ball would have a huge impact, without requiring any special technology or chemicals unknown to the era. The problem with chemicals would be, that even if you knew what it was, you could probably not locate it and start mining it, and even if you managed, you would need a large infrastructure to make use of it.

Assuming you could get into a position to explain your ideas to someone with enough power, you could make their guns much more effective with historical knowledge and very little to no new technology.

Standardizing calibers would make their guns much lighter, easier to load, and less prone to misfire, besides making logistics easier. They had rifled barrels in the early times, but they didn't use it much because it was a nightmare to load, prone to misfire, and you needed to clean your gun after almost every shot, or the residue caused the next bullet to be stuck in the barrel. With the ingenious idea of the Minié ball, you could make their gunners much more effective, by significantly increasing both their firing rate, range, and accuracy.