Gun-metal is an alloy of copper and tin, with sometimes a small proportion of zinc. It is a harder metal than either of its constituents and has a greater density. It is more fusible and less likely to corrode than copper. When a casting of gun-metal rapidly cools or chills, the density, strength, and toughness of the metal are increased because the composition becomes more uniform. For heavy bearings hardness is considered of greater importance than strength. A hard gun-metal consists of 79% copper and 21% tin.

The general effect of tin in an alloy is to increase its hardness, and whiten its color. Zinc in small quantities with copper increases fusibility without reducing hardness; in large quantities it prevents forging when hot, but increases malleability when cold. Common bronze mixes better when a small quantity of zinc has been added.