This is not just a recommendation. So far, The isekai where modern/sci fi weapons exist in fantasy world. Name many. Gate Jietai 1) Gunota 2) Magic Bullet in Magic Land 3) Release that Witch 4) including original stories: Out of Space 5) There are shit ton of stories like these, name some and rank them from best to worst.
Arifureta Second Coming of Gluttony Modern weapons cheat in a fantasy world Infinite Competitive Dungeon Society Manuke FPS GM logs-in to another world Heroes of marvel Overgeared Deathmarching in a parallel wotld rhapsody
A Hero's War Tales of Paul Twister series Akuyaku Reijo Ni Koi Wo Shite Tales of the Reincarnated Lord Bu Sha aka Kill no More Zero no Tsukaima All of those are isekai Web Novels
Do far er only have Heroes WHO have guns and firearm, but even og the bladguld her Access to Them it wont chance lol.
I forget the name but its something along the lines of A Gun Nerd That Reincarnated Into Another World or something along those lines
I suppose Lord of the Mysteries counts as Isekai with guns, but you can't really call it modern/sci-fi weaponry, it's 19th century tech at the latest.