The W40 nuclear warhead was an American nuclear warhead developed in the late 1950s and which saw service from 1959 to 1972. It was a fusion boosted fission bomb type. The W40 design was reportedly the common Python primary or fission core used by the US B28 nuclear bomb, W28 nuclear warhead, and W49 nuclear warhead. The W40 was 18 inches in diameter, 32 inches long, and weighed around 385 pounds. It had a design yield of 10 kilotons. The W40 was used in the MGM-18 Lacrosse surface to surface missile (SRBM) and Bomarc Missile Program surface to air missiles (SAM).