Monster Hunter: World - 15 Best Tips & Tricks On The Internet
6. Why Can't I Use X Ammo/Coating?
Simplified in comparison to past Monster Hunters, the exact uses of ammo and coatings is still likely to confuse, as they show up in the Item Box with a whopping great red "X" overtop.
This simply means that your current weapon isn't compatible with that ammo type, but even if you have a Bow and a particular coating is still crossed out, then what?
It means that particular Bow isn't equipped to handle that type of coating, and you'll need to visit the Smithee to purchase an upgraded version, or upgrade your existing weapon until it can. Pay attention to the bottom right ammo/coating list when hovering over any new Bow or Gun, as this will tell you which ammo types are going to be compatible in the field.
Even after this, once you're back in the map, you'll need to ensure each ammo type is in your ammo pouch, then use up and down on the D-pad to select them.
Phew.