Shootout is a Justice deck specific card added in the big Beta 6.6 Update. Previous Versions can be found here.
This card deals 1 damage to everyone on the field for 3G. This is useful if you want to get rid of your Mettaton NEO if you don't want to waste a Punishment and don't have any other important monsters on the field.
It can also be useful as an alternative for Strafe when you want to combine it with Asgore. Just keep in mind that Diamond Boy 1 will cancel this for its allies.
With its change in Beta 16.0, it will trigger again whenever it kills a monster. This can be useful if there are some 1 HP monsters on the board like Temmie or Ice Cap in order to deal more chip damage to all other enemies. Keep in mind that you and your monsters also take damage with this.
Another crazy combo is by having at least 3 Big Bobs on the board, with at least one with 1 HP and one with 2 HP. By using this card, the result ends up dealing 15 damage to everyone, as shown here.