Douglas Harper's Etymology Dictionary
gun-metal
commonly an alloy of copper and zinc; used attributively of a dull blue-gray color since 1905.
Usage examples of "gun-metal".
The gun-metal sky lightened and glowed red along the horizon until, like a thermic lance, the sun tore a white-hot hole in it.
The smiling officials inside the hut were now wearing kitschy lederhosen shorts and perky hats as they greeted a visitor - but the gun-metal eyes were the same.