Basic design concepts for gun boosted rockets are discussed and the launch parameters of the gun examined. The design of the Martlet 3A subcaliber rocket with six-inch motor for launch from the Barbados sixteen-inch gun is described as well as the firing results obtained in the period September 1963 to January 1964. Two successful flights with motor ignition were made with maximum acceleration loadings of 6,500 gs and 3,500 gs respectively. The design of the larger Martlet 3B with an eight-inch motor is described with its test firing series in July 1964. These tests showed that center cavity rocket grains are not suited for gun launched projectiles. The use of end burning rockets or fillers in the center cavity during gun launch is indicated.