W54 (nuclear weapon)
Now retired, W54 nuclear weapon was the lowest-yield weapon ever deployed by the United States. It was best known as the "backpack" special atomic demolitions munition (SADM), but was also used as the warhead of the AIM-26 Falcon air-to-air missile and of the Davy Crockett artillery weapon. Its yield, depending on the model, was in the 10 ton to 1 kiloton range.
The SADM version is most historically significant, as it is one point of reference for the potential of a portable nuclear weapon, and the unlikely existence of "briefcase" implementations. The warhead itself weighed 58.6 pounds, and a complete assembly, in its shipping container, weighed "less than 163 pounds". Since it could be carried, as a backpack, by a single paratrooper, the fielded version, removed from the container, probably was in the 100 pound range.
As the Davy Crockett version, it was a 51-pound warhead, fired from a 155mm recoilless rifle, mounted on a tripod or on a jeep. The yield was 18 tons.  The 2.5 mile range of the recoilless rifle made its use tactically impractical in any defensive mode; it would have to be so close to the enemy that it could easily be overrun before permission was granted for its use.
Safeguards and surety
The SADM was reported to have only a combination lock and timer as a safeguard.
- Thomas Cochran, William Arkin, Milton Hoenig (1984), Nuclear Weapons Databook, Volume I: U.S. Nuclear Forces and Capabilities, National Resources Defense Council
- W54, Globalsecurity.org
- Sublette, Carey (20 February 1999), 4.2 Fission Weapon Designs, Section 4.0 Engineering and Design of Nuclear Weapons