You're underestimating just how high up metallic cartridges are in a technology base. We knew about them since before the revolutionary war but the actual techniques and technologies to make them viable didn't appear until nearly 100+ years later. Hell, paper cartridges existed as a stop gap technology for many weapons and those had numerous downsides as well. That's not even getting into the science of ballistics and ensuring that each cartridge is within allowable tolerances (typically between +0.05/-0.05 for sloppy fits, +0.005/-0.005 for typical ones) since going out side of those risks jamming the gun or the round detonating out of battery... and if I remember correctly, in the LN, they had AKs and a Barret .50 cals in that scene (which makes the sisters shooting them standing even crazier since those things weigh about 35-40lbs and generate a fairly sizable amount of recoil).
Someone already posted in here, but the technologies you need for metallic cartridges are:
Case shaping
Bullet casting (for simple lead ball)
Primer manufacturing (this is the huge one. Without primers, metallic cartridges basically aren't feasible)
And gunpowder manufacturing (easiest but also the most dangerous. Also depends on whether or not you make smokeless powder or blackpowder. Using smokeless in a blackpowder weapon is a surefire way to experience a spontaneous disassembly of the weapon)