Could you give me a brief summary of how little they use guns, I'm not a Warhammer fan
Long story short, man got buttfucked by sentient AI. GW stole the Terminator script and copy paste the entire thing. So now, improvement to tech is BAD, praying to a fictional god of machine GOOD.
Kind of why they have laser tag guns and bolters that shoot explosive soda cans, but nothing stronger.
Here is my plot with guns in medieval times. They exist but like martial arts and magic cultivation techniques, they are heavily suppressed and kept by those with power because the last revolution was by the peasants who found these powers and after that, they know how dangerous these thing are and hypocritically sealed them away while they turned from the oppressed to the oppressors.
Also, gun manufacturing is quite expensive without the right tool and when the higher ups are busy suppressing the industrial revolution, it becomes even more impossible to manufacture these weapons en masses.
Notice that I say en masses. Because in my setting, guns can hurt or even kill pugilist and mages like the common knife when used as a surprise weapon. They are also used by these empowered folks as weapons too, especially when they can have funds and access to the tech. Then the story goes with both the pugilists and mages developing anti-projectile skills, like shifting winds around their bodies to deflect or block bullets that are travelling slower than mach 1 (most old gunpowder weapons' projectiles are that slow and peasants only have access to those) or even straight up out run the bullets.
Or you can copy Dune's body shield that blocks high speed projectiles.
For your setting, you can limit gun uses but not preventing cannons and artilleries, if the caliber is big enough and fast enough, any excuses of not using guns are moot.
Also, like you said for magic bullets, they are expensive and cannot be used extensively in armies. Maybe a trained group of snipers and assassins but not to the common soldiers, they are not worth that much. Also, is your army a bunch of superhumans who can benchpress cars? If yes, then a gun is pointless unless it can provide extra damage. If not, you can state things like battles are determined by the strong freaks and they fear no guns.
Also, Nanolaminate Armor, Phase Shift Armor types that makes small projectiles useless unless at close quarters, defeating the purpose of a gun.