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The W9 was 11 inches (280 mm) in diameter, 54.8 in (1,390 mm) long, and weighed 850 lb (390 kg). It had an explosive yield of 15
The W9 was a gun-type nuclear weapon, using around 50 kg (110 lb) of highly enriched uranium in one large rings assembly and one smaller bullet, which was fired down a tube by conventional explosives into the rings assembly to achieve critical mass and detonate the weapon.
The W9 units which were retired in 1957 were recycled into lower yield T-4 Atomic Demolition Munitions. These were the first (semi) man-portable nuclear weapons.
The W9 is only the second gun-type nuclear weapon known to have been detonated; the first was the Little Boy nuclear weapon used in World War II.
The W9 artillery shell was test fired once, fired from the "Atomic Annie"