The attachment is available for the EM1, the EPM3 and the AE4. It allows the weapon to be fired for a longer amount of time before it overheats. It is integrated to the EPM3 "Panorama" supply drop variant.
The Heat Sink allows the equipped weapon to be fired for longer than usual. It decreases the rate of heat accumulation in the weapon by 25%, raises the rate of heat dissipation while reloading the weapon by 25%, and decreases the time penalty due to a complete overheat of the weapon by 25%.
For example, the Heat Sink allows the EM1 to fire for 5.35 seconds instead of the default 4.00 seconds; the EPM3 to be fired for 11 consecutive shots, instead of the default 7; and the AE4 to be fired for 25 consecutive shots, instead of the default 20. It is unlocked by achieving three double kills with the EM1 and the EPM3, and by achieving 20 hipfire kills with the AE4.
In Exo Survival, the AE4 cannot accept the Heat Sink attachment.