Automatic
Source Player Core pg. 255
In addition to a normal Strike, you can fire this weapon using the Auto-Fire action.
Paizo Clarification
If there are more targets in the area than ammunition expended, only creatures up to half the ammo expended from the weapon need to roll Reflex saves. Which creatures are decided upon by the GM based on positioning and the weapon used, but generally they are the creatures that are furthest away.