A composite barrel assembly for a gun, such as a muzzle-loading mortar for fin-stabilized projectiles. The composite construction of the barrel has a plurality of generally concentric layers built-up around a rigid supporting liner which may be fabricated from any of several metal or ceramic compositions. An inner thermal barrier coating of inorganic glass, metal refractory alloy, chromium alloy, functionally graded material or ceramic is disposed within the liner. An overwrap layer of continuous fibers embedded in a matrix surrounds the liner. The matrix is either a resin, polymer, ceramic, glass or metal. An outer shell of continuous fibers embedded in a high temperature polymer matrix surrounds the overwrap. When the liner is made from a metal-based composition, an outer thermal barrier coating may be applied in between the liner and the overwrap.