Metal Gear Acid

Need to use a gun that shares the same bullet type on it. check details on your cards to find the bullet types.

To clarify that answer, the key is to have multiple guns of the same type, or same ammo type in your deck. Once you have a weapon equipped to one of your slots you must wait until you have another compatible card in your deck, then select equip and move it to the slot already filled. If the guns are indeed compatible, the on-screen text will read Load; if not the weapon will replace the one already there, thus wasting a turn. Also keep an eye out for special cards, such as Machine Gun Kid, which have the ability to load multiple weapon types.