 The gun had been thoroughly tested and was well past intercontinental ranges, but needed modifying.In early 1963 HARP started experimenting with the "Martlet-3", a 7-inch-diameter (177.8 mm) "full bore" projectile designed to test the basic problems of launching a solid-fuel artillery shell from guns.Solid shell fuel has the consistency of soft rubber and is cut into a pattern that is open in the middle, so on firing the "grain" would tend to collapse into the cavity.