Gun-Ota ga Mahou Sekai ni Tensei shitara, Gendai Heiki de Guntai Harem wo Tsukucchaimashita!?, Doushitemo Hametsu Shitakunai Akuyaku Reijou ga Gendai Heiki wo Tenishita Kekka ga Kore desu Recommendations
Both of them uses their modern knowledge to build and use firearms in a fantasy world.
Gun-ota explains the application of magic and knowledge of firearms in a more detailed way. While the other one not as much as Gun-ota but is still quite fine for a quick read.