gunmetal, also called G Metal, variety of bronze, formerly used for
ordnance. Modern admiralty gunmetal is composed of 88 percent
copper, 10 percent tin, and 2 percent zinc and is used for gears and
bearings that are to be subjected to heavy loads and low speeds. It
withstands atmospheric, steam, and seawater corrosion and is suitable
for valves, pump parts, and steam fittings.
from Encyclopedia Britannica