A thermal sleeve is a device around the length of a gun barrel of a large caliber gun, typically found on modern tank

Tank

A tank is a tracked, armoured fighting vehicle designed for front-line combat which combines operational mobility, tactical offensive, and defensive capabilities...

s. 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

Royal Ordnance L11A5

The Royal Ordnance L11A5 is a 120 mm L/55 rifled tank gun design. It was the first of NATO's 120mm tank guns which became the standard calibre for Western tanks in the later period of the Cold War...

 used on the Chieftain tank

Chieftain tank

The FV 4201 Chieftain was the main battle tank of the United Kingdom during the 1960s and 1970s. It was one of the most advanced tanks of its era, and at the time of its introduction in 1966 had the most powerful main gun and heaviest armour of any tank in the world...

.