An evaporative barrel cooler for a firearm includes a firearm adapter having a mist nozzle, and a high pressure coolant reservoir attached to the firearm adapter with a flexible hose. The firearm adapter orients the nozzle inside a firearm chamber and aligned with the bore of a firearm barrel. The firearm adapter conforms to the interior of a firearm action and replaces a firearm bolt. The firearm adapter accesses the chamber through an open action port or through the bolt's raceway. The high pressure coolant reservoir may further include a hand pump attached to the reservoir for pressurizing the coolant. A coolant valve may be incorporated into the flexible hose to control the flow of coolant, whether it is water, alcohol, gunpowder or bullet metal solvent, or a nonflammable and nonreactive material.