Why rapid firing blisters paint on naval gun barrels

Did the blistered gun barrels photographed after bombardment reflect overheating or a particular construction method?

The visible blistering was usually protective paint lifting under heat, something any rapidly fired gun could show, rather than the steel barrel itself blistering.

Naval gun barrels were painted to protect them from salt water. Sustained firing could heat the barrels enough to blister or burn away that coating. Apparent blistering in photographs therefore did not necessarily mean the barrel steel itself was damaged or that a particular construction method was at fault. Ajax and Achilles returned from the River Plate action with blackened barrels after firing large numbers of 6-inch rounds.