Some weapons, such as machineguns, can overheat if fired continuously too long. When this happens, the gun locks up until it cools down enough to fire again.
This property adds heat to heat bar per round fired. This value is the fraction of the total heat bar, not a fraction of a bar. When the heat reaches 1.0 or greater, the weapon is overheated for TimeDelayOnOverHeat seconds. In the .con files you will see values like 0.03, 0.04, 0.05 for this value. The Calliope Sherman tank uses 0.13 per shot, so 8 shots would overheat it; in practice, 9 shots overheat it, as the CoolDownPerSec rate cools the weapon (even when firing).
This property affects the heat bar, but the heat bar itself can have a different meaning if VelocityDependentOnHeat is set to 1. When that property is set, the amount the heat bar reaches determines the velocity of the object, i.e. how far you throw the grenade. A right-click-hold when using a grenade builds up this "heat" until release.
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