See title. I couldn’t figure this out at first and the ingame tutorial is not very clear, so I’m leaving this here for people who may also be confused.
Guide to Use Box Weapons
These are any weapon with a “Bx” at the end. They are all shown on the last page of the Customize menu.
In the top-right of the screen, a box will appear and start filling up when you have a Bx weapon and are actively shooting (not holding down the action button).
You have to be holding down the action button (z on keyboard or L/R/A/Y on controller) at the moment the box completely fills up in order to activate the weapon.
If you are holding the action button, the boxes will stop appearing. So it’s best to press it for a short time as the box is about to be full. This is to the beat of the music and depends on the specific Bx weapon, you will get a feel for a specific weapon as you use it.
Good luck have fun!