Gun-Ota ga Mahou Sekai ni Tensei shitara, Gendai Heiki de Guntai Harem wo Tsukucchaimashita!?  

When a Gun Nerd Is Reincarnated to a World of Magic, 軍オタが魔法世界に転生したら、現代兵器で軍隊ハーレムを作っちゃいました!?

Houta Youta was 27 years old, a virgin who dropped out of high school and spent time as a hikikomori. Despite facing difficulties throughout his life, he met an untimely end when he was stabbed to death on the way home from work at a metalworking factory. However, it appears that he has been reborn in another world with different circumstances.