Actually, casting bullets from lead isn't difficult at all (simply extract a bullet from its' cartridge and use it to make a mould to cast fresh ones out of lead).
The tricky parts are recovering the brass casings, and making fresh primers.
The brass casing is an essential part of modern firearms. It mechanically holds all the parts together (bullet, powder or propellant and primer) in the proper mechanical alignment to work with all the other mechanisms of the firearm. It provides protection from the elements and temperature extremes while the rounds are being transported, and upon firing, the brass is flexible enough to expand slightly to seal the chamber (a modern rifle can produce 50,000 lbs/inch^2 of chamber pressure), then contract as the pressure drops, and carries a lot of the heat from the chamber on extraction and ejection.
Drawing the brass with enough precision to make an acceptable case might be possible with medieval technology, although I would have doubts. The link above shows the process in some detail, which suggests that the high precision cases needed by modern weapons are not going to happen, instead each case must be painstakingly hand crafted to ensure a proper fit.
One other issue you haven't really addressed is black powder isn't very effective as a propellant compared to modern formulations, and is dirty and corrosive to firearms. Weapons and even automatic weapons have been made to use black powder. Early Maxim guns used black powder cartridges, and earlier "cranked" weapons like Gatling and Nordenfelt guns also had been made for black powder, but remember these were designed to use black powder and had greater operating clearances and highly trained gun crews who stripped and cleaned the weapons on a regular (daily) basis under the supervision of the Sergeant. M-16 rifles during the Viet Nam war were plagued by problems because the solders were issued ammunition with a slower burning "powder" which fouled the weapon, and were (amazingly enough) told they did not have to clean the weapon. This problem ended with ruthless cleaning and inspections by the unit NCO's.
Three barrel Nordenfelt gun on a field carriage
Of course, there must be a reason to use firearms rather than adopt local weaponry. Building steel crossbows would be far easier and more efficient, since it allows the heroes to arm virtually everyone in the village with an effective weapon (capable of felling knights), and providing a large numeric edge over the highly trained and very expensive fighting men (knight and Men at Arms). This is the essence of the Infantry Revolution of the 1400-1500's, where weapons and tactics were developed to allow large numbers of men with limited training to effectively contest the battlefield against smaller numbers of highly trained warriors.
lots of these guys make up for lack of firearms