FUCK FINALLY SOME TECH IN AN ISEKAI WORLD. That being said, the king is a massive medieval larper. And I'm not sure if those armor designs are practical for wielding guns given the visor obscuring the wearer as well as the lack of places they can store ammo and potentially gunpowder. The best armor is to not get hit or else the knights are dragging themselves down. However given that they somehow have CARS and TRAINS the excuses become more limited. Probably means that they use magic to fuel what looks like flintlock rifles like One Piece or else 90% of their time is spent reloading.