I am a world-hopping warlock who regularly goes on adventures in various worlds with differing levels of technological knowledge. Being a crafty warlock, I have a number of tricks, weapons, and gadgets up my sleeves, and I'd like to add firearms to my repertoire.
I'm currently planning an extended campaign in Acardia, a continent with a technological and political structure comparable to what you folks refer to as early medieval Europe. My early scouting missions haven't discovered any gunpowder based weapons, although I can't confirm that no such thing exists there.
Once the campaign starts in earnest, It won't be feasible to return to a more advanced world for supplies. In addition, I'll be traveling a great deal, so I don't want to cart heavy machinery around with me. I'd like to utilize firearms as a form of self defense during this campaign.
Here's the thing, though- I don't want to be that idiot who thinks they can conquer an entire continent with a single machine gun. If I'm going to use firearms in this campaign, I need to take into account the manufacture of ammunition, repair of the weapons, etcetera.
So... what's the most advanced sort of gun I can take with me and be able to utilize for the entire campaign, which could last for years? It's acceptable if the guns themselves can't be manufactured using the tools available, so long as they can be maintained using Acardian tech. I will, however, need to manufacture ammunition. I would also prefer to be able to fire more than once before reloading, and use bullets that include powder inside of them rather than being loose metal balls (if possible). (If I'm correct, guns that use such bullets tend to be easier to clean.)
I'll be campaigning with a small team, no more than 20 to 30 people. I don't currently have a motor vehicle that would be practical to fuel and maintain during the campaign, so I'll probably be using horse-drawn wagons to carry anything I need to take with me.
If the tools required to make ammunition or a part can themselves be easily made with Acardian tech, that is also acceptable, but keep in mind I won't have the resources to start an industrial revolution. If the equipment in question is too heavy to cart around, and I can't make new ones when I enter a new city, that will put a damper on it, but it won't necessarily completely invalidate the technique in question.
I'd also like to note that, given the choice between power and accuracy, I'd probably go for accuracy, although it would certainly be nice if the gun had the power to punch through steel plate.