Gunmetal

The beſt Guns are not made of malleable Metal, and cannot be made of pure Copper or Braß; but it is neceſſary to put in courſer Metals to make it run cloſer and ſounder, as Lead and Pot-metal. […] About 20l. of Lead is uſually put into 100l. of Pot-metal; but about 6l. is ſufficient to put into 100l. of Gun-metal.

Source: wiktionary

Braſs, Copper, Latten, Bell-Metal, Pan-Metal, Gun-Metal, or Shruff-Metal, whether clean or mixed, carried beyond Sea, forfeits double the Value thereof; (Tin and Lead only excepted) the Informer half, […]

Source: wiktionary

Phosphorus is known to improve the strength of copper, and to make it cast soundly. Abel, chemist to the British War Department, stated before the Institute of Civil Engineers, that he had made some experiments upon the combinations of phosphorus and copper, and "had found that by the introduction of a small proportion, say from 2 to 4 per cent., of phosphorus into copper, a metal was produced remarkable for its density and tenacity, and superior in every respect to ordinary gun-metal. […] The experiments alluded to were merely preliminary, and had been, to a certain extent, checked by the improvements since introduced in the construction of field-guns, which had led to a discontinuance of the employment of gun-metal."

Source: wiktionary

The pump [of the steam fire-engine], being at the forward end, was made of gun metal and was double acting, having eight brass suction-valves, four on each side, and four discharge valves, opening into the air vessels, to which a double discharge valve, having one opening on the inside and two on the outside, was attached for the purpose of using two lines of hose.

Source: wiktionary