A thermal sleeve for a gun barrel is in the form of a cylinder of a substantially rigid thermally insulating material (2,4,6) with a series of longitudinally and circumferentially spaced spacer members (20) disposed on the inner face of the cylinder. The spacer members (20) engage with the gun barrel and define an annular gap between the cylinder and the barrel. The sleeve (18) further includes means (28) for sealing the annular gap as well as means (32) for securing the cylinder on the barrel to ensure pressurised contact of the spacer members (20) with the barrel.