A thermal sleeve, or blanket, is a device around the length of a gun barrel of a large caliber gun, typically found on modern tanks. Its primary purpose is to provide a more consistent temperature to the gun barrel preventing distortions due to thermal expansion caused by the temperature differences around the barrel when firing.^[1]

Thermal sleeves were originally simply insulators. They would prevent ambient conditions such as bright sunlight or winds from heating or cooling one side of a barrel more than the other which would cause a thermal distortion (bending or drooping), reducing accuracy. More modern variants contain concentric inner and outer insulating sleeves with a gap in between. Versions have been created which are detachable from a given barrel so that they can be re-used with a replacement barrel. Proposals exist for types that have advanced external thermal and radar profiles, reducing their thermal and radar signatures making the barrel and thereby the tank harder to detect.^[2]

One of the earliest guns to use a thermal sleeve was the Royal Ordnance L11 used on the Chieftain tank.