...
When shooting for short periods of time, heat build-up usually isn't an issue, and primary guns DPS is the "Burst Weapon Output" listed in the ship purchase/upgrade menus. When fired for longer, especially when several are fired together, heat can force primary guns to shutdown temporarily. When firing over long periods leading to overheating (and weapon shutdowns), primary guns DPS is reduced to the "Sustained Weapon Output" (please note that at this time, the ship purchase/upgrade menus lists incorrect sustained MW values).
Minor heat buildup (short bar the on right is short)
Heavy Heat buildup (bar is long, and orange)
...