I’m currently working on something where breechloading rifles and cartridge shot are developed very quickly over matchlocks and flintlocks. What major technological hurdles need to be overcome for breechloaders and bullets to be present? They don’t need to be mass produced or very common, but in use for small numbers of special military units.
The weapons may be made by artisans, but the ammunition absolutely needs to be mass produced, otherwise the weapons become ineffective curiosities.
Casings and primers are mandatory. Casings need late 18th century metallurgy, mechanical engineering and manufacturing organization, or else they would be prohibitively expensive. (They could make them in the Middle Ages, but the effort would have been more similar to jewellery than to ammunition.) Primers need early 19th century chemistry.
Smokeless power is highly desirable. Mid-19th century chemistry.
Good quality steel with reproducible manufacturing process is also highly desirable, unless you want you breechloaders to be made of bronze. Late 18th century metallurgy again.
In real history, breechloading fire arms appeared about 200 years after flintlocks. Whether this is "very quickly" enough or not depends on the story.
There is a well-known alternate history series, beginning with Eric Flint's 1632 (2000). One of the most entertaing characteristics of the series is that all technological developments are quite well researched, and staged in a highly plausible order. You may want to look it up.
There are a few problems with your premise:
breechloading rifles and cartridge shot are developed very quickly over matchlocks and flintlocks. What major technological hurdles need to be overcome for breechloaders and bullets to be present?
The problem is metallurgy. You need to make BRASS cartridges, as it's found to be the most efficient way to create a "round", which is composed of the bullet, shell, propellant, and primer. Assuming we're talking about centerfire or rimfire cartridges. (Anything else is too weird or unreliable). Brass is the easiest to make. Anything else can't stand the heat nor the force involved. But achieving a good "seal" to ensure the explosive power is only vented ONE way is NOT that simple. You don't want it so tight you can't eject the spent cartridge, but you need it tight enough so the hot gasses don't blow back in the shooter's face.
Muzzleloaders are easy to make because there is no breech. No open portion that has to fit right and withstand the pressure. On the other hand, they are impossible to load while prone. You'd need proper metallurgy to advance to breech loaders, which also goes with the ammo.
Black gunpowder is also corrosive and requires cleaning after every X shots (depending on the design) or the gun will be gunked up and stop working right. Smokeless powder burns much cleaning and eliminated most of the problems with black powder.
They don’t need to be mass produced or very common, but in use for small numbers of special military units.
The guns may be crafted by artisan-craftsman, but not having enough of them makes them pretty useless overall. Why issue them at all? And you'd need LOTS of ammo. Having 10 rounds makes them so scarce that people wouldn't fire them because they are so worried about NOT using them right. But if you have 50 rounds, you'd be less careful about hoarding them.
You'd at least need enough to equip a unit, like a platoon or even a company of the soldiers so they can perform volley fire, and that uses a LOT of ammo. Imagine 3 ranks of troops, one prone, one kneeling, one standing. Every few seconds, a volley of bullets goes out. By the time the 3rd rank fired, first rank's reloaded. This repeats until they annihilated everyone in front, or they run out of ammo.
some early cannons on sailing ship were breach-loading, which means there should technically be no reason why your breach loaders couldn't have been made with the technology of the time period.
it'd be a "Latch-action rifle" akin similarly to bold action rifles, and you could use a similar setup to how WWI and WWII navel cannons in that you also use the latch at the end of the breach to load the ball, and have the black powder propellant be in a tiny cloth bag. the Russians in WWI actually used black power as the propellant for their naval guns on the predreadnought battleships from what I had read. and all warships the world over had their propellant in separate bags that were loaded in right after the shell was loaded into the breach by the power rammer on the loading tray.
so i do hope this could be of help to you