Nagato Kazuya, a "Military Nerd" high school student who gains the ability to summon modern-day weapons and soldiers, when he reincarnated into another world. Together with his summoned yandere military advisor, soldiers and modern-day weapons, they will conquer the land, seas and skies of a fantasy world.

This is a Kingdom-building manhua its very fun and interesting MC gets Isekai-ed and gets a system to become strong and recruit strong people using gacha and other system functions.

Kill the villainess - after being isekai'd MC attempts suicide several times hoping it'll send her back to her original world, then the whole plot is her trying to figure out a way to go home. This! KTV is the only isekai (that I've read) that actually focuses on how much being forcefully transferred can fuck you up mentally and emotionally

Gunslinger Girl (the first one) - the Italian Social Welfare agency trains cybernetically enhanced young girls, then brainwashes and trains them to become assassins. Jormungand features a child soldier and an arms dealer so plenty of guns. MC of Mahouka uses guns but they are not actual guns which shoots bullets.

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isekai manga where mc uses guns