I’m looking for a manga where a female main character spawns into or is reincarnated into a fantasy world. She uses guns to kill monsters and they drop token/crystals that she uses to exchange for more ammunition and better guns and stuff like that.
The lead does not use guns exactly. But, "Itai no wa Iya nano de Bōgyoryoku ni Kyokufuri Shitai to Omoimasu / Bofuri: I Don't Want to Get Hurt, so I'll Max Out My Defense" pretty much matches your description.
The premise is basically explained in the title itself. Urged on by her friend, the main character begins playing a VRMMORPG online. Being an amateur gamer she does not want to get hurt. Hence, she opts to be a shield user with maxed out defense stats and continues putting every status point she earns in the game into increasing only her defense level.
As a result, she is left with low agility, no magic, etc. but her high defense allows her to endure most attacks without taking much damage. Along with her moderate creativity and the help of her friend, she makes unexpected accomplishments in the game, its quests, and events. By doing so, she ends up earning all kinds of equally unexpected skills and becomes one of the strongest players in the game.