Jupiter Hell uses a turn-based grid system, so it will automatically target enemies that come in sight. The target that is chosen will have green edges around the square grid space they are occupying. So, upon opening fire, that target will be riddled with your bullets.
However, there will be certain situations that the automatic target is not the best one to be shot to deal with the situation at hand. There may be several enemies surrounding a red explosive barrel or taking down the enemy with the shotgun first before the one wielding the pistol.
Related: All Jupiter Hell classes explained
To manually aim, you need to press the Right Thumbstick in or T on the keyboard. This will highlight a square with a white target outline. You can move this target, and the spaces underneath will turn green. Upon moving the white target to the point, you want to shoot, aka the red explosive barrel. Press X, click Right Thumbstick, T, or F again, which will fire the shot at your chosen location.
The green highlight also indicates the spread of the weapon you have equipped. The pistol has no spread, so the highlight is in straight lines. But the shotgun has a large spread, so you can see the area your bullets will hit, meaning you can see how many enemies you will hit upon manually aiming.
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