A thermal sleeve 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 prevent the gun barrel from being thermally distorted due to environmental conditions, especially when the barrel is already hot due to frequent firing.
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.
One of the earliest guns to use a thermal sleeve was the Royal Ordnance L11 used on the Chieftain tank.