I use an electric air pump for inflatables ($20). It has an on board rechargeable battery and I attach the output to some rubber tubing. I bring lunch bucket cooler with some ice in it. When I'm ready to cool the barrel, I put the downstream end of the rubber tube in the chamber, turn the pump on, set it on the ice and close the cooler lid (of course it doesn't close completely because the air hose is coming out of it). It actually makes the barrel cold, first at the breach and then at the muzzle.