What anime has a gun that is loaded with elemental cartridges?

You're likely thinking of Black Clover, which features a character named Yuno who uses a grimoire (a magical book) that enables him to wield wind magic.

While Yuno doesn't technically use a gun, his grimoire grants him the ability to shoot blasts of wind that can be enhanced with elemental cartridges.

Here's how it works:

* Yuno's grimoire has a unique ability to transform mana into bullets.

* Elemental cartridges are used to impart different elemental properties to these wind bullets, such as fire, water, earth, etc.

So, while Yuno doesn't use a traditional gun, the mechanism of his grimoire and its compatibility with elemental cartridges make it a fitting answer to your question!