The 4000-lb high-capacity design was little more than a cylinder full of explosives: it was unaerodynamic and did not have fins. By comparison the similar American "4,000 pound LC Bomb AN-M56" bomb was aerodynamically designed as other US bombs were, with a sheet metal tailfin assembly and shaped nose and aft sections. When fitted with a conical "nose piece" and a drum tail, the British "Blockbuster" bomb fell straight down. These bombs were designed for their blast effect, to cause damage to buildings, specifically to blow roof tiles off, so that the small incendiary bombs could reach the building interiors. In contrast to the American AN-M56 ordnance, the cylindrical "HC"-class British-design high capacity bombs were used only by the RAF, which was the only air force with bombers with bomb bays large enough to hold them.