The barrels come in a variety of weights and are usually constructed from brass, silver-nickel, or a tungsten alloy. Brass is cheap but light and therefore brass barrels tend to be very bulky. Tungsten, on the other hand, is twice as dense as brass; thus a tungsten barrel of equivalent weight could be thirty percent smaller in diameter than a brass one. Pure tungsten is very brittle, however, so an alloy is commonly used, with between 80 and 95 percent tungsten and the remainder usually nickel, iron, or copper. Silver-nickel darts offer a compromise between density and cost. Barrels come in three basic shapes: cylindrical, ton, or torpedo.