The largest conventional bomb currently in U.S. military service (the GBU-43/B MOAB informally known as the "mother of all bombs") introduced to the U.S. arsenal in 2003 and intended to be delivered via a modified C-130 military cargo plane (specifically an MC-130E Combat Talon I or MC-130H Combat Talon II) and has an explosive yield of 11 tons of TNT and weighs 22,600 pounds. Fifteen have been made but none have been used in anger. It is too large for use by U.S. fighter and bomber aircraft.
MOAB is a successor to the the BLU-82 Daisy Cutter, a very large bomb (although not quite so large) used to clear jungle for helicopter landings during the Vietnam War, and was used a number of times to shock and awe large deployments of infantry in Afghanistan and Iraq and to kill opponents in cave complexes (although it is not strictly speaking a "bunker buster" bomb). The only larger bomb in military service nicknamed the "father of all bombs" is in Russian military service and reputed to have a yield about four times a great.
The explosive yield of MOAB is roughly equal to the explosive yield of the smallest tactical weapon every put into service in the U.S. which approaches the technological minimum size limit for a nuclear weapon.
Most bombs in U.S. military service have yields of one ton of TNT or less.
The Smallest Tactical Nuclear Weapons
Tactical nuclear weapons are those intended to be used in military battles or for discrete objectives other than obliterating cities or regions entirely.
The Mk-54 Davey Crocket "recoilless rifle" round was used together with an M-28 120mm launcher with a 2 km range, or M29 155mm launcher with a 4 km range (basically mortars operated by a crew of three from a Jeep or armored personnel carrier) was first produced in 1956 and was in U.S. military service through 1968 at the height of the Cold War had a 51 pound W54 variant nuclear warhead as part of a 76 pound mortar round (excluding the launcher) that was 31 in. long with a diameter of 11 in. at its widest point. This round was "very close to the minimum practical size and yield for a fission warhead." About 2,100 of these rounds were built.
The Mk-54 had a minimum explosive yield of approximately 10 to 20 tons of TNT, at the low end, about the same as MOAB but was not very accurate and in testing sometimes produced a larger than intended blast. It would have delivered a fatal radiation dose to everyone within a 400 meter radius of the blast location in addition to a large explosive blast that flattened area area equal to about two city blocks and leaving residual radioactivity. A test of this weapon in 1962 was the last atmospheric test detonation at the Nevada Test Site.
It is apparently technically infeasible (although given advances in technology since the last tactical nuclear weapons were designed this may only be for lack of military or political interest in this technological challenge) to make a tactical nuclear weapon so small that it has only a 1 ton of TNT yield about the size of a typical conventional bomb dropped by a U.S. fighter or bomber. If it was feasible to do so and the weight of the weapon scaled to its explosve force, a nuclear bomb with that yield would be about the same weight as a gallon of milk.
U.S. military doctrine called for these weapons to be used to close "chokeholds" like the Fulda Gap mountain pass in East Germany or tunnels or bridges that could have provided access points for Soviet tanks into Western Europe. No tactical nuclear device has ever been used in combat.
Variants of the same warhead were also used in three other weapons systems:
The SADM was carried by a team of two special operations paratroopers in a backpack to the place were it was to be detonated to destroy major infrastructure and set on a timer with the soliders who delivered it leaving before it exploded. It was cylinder about 16 by 24 inches) that weighed 150 pounds with a yield of 10 tons to 1 kiloton that could be adjusted (at least in theory) by the operator. The other two variants were mounted in missiles on fighter aircraft
The two nuclear bombs that were used by the United States against Japan were dropped in Hirokshima (13-18 kilotons) and Nagasaki (20-22 kilotons) at the close of World War II, each of which destroyed an entire central city city with a single bomb, were roughly a thousand times more powerful than the Davey Crocket rounds. These are the only nuclear weapons that have ever been used for a purpose other than a nuclear test by any country. Subsequent nuclear weapons in U.S. military service were designed with yields of 100 to 25000 kilotons. "As a comparison, the blast yield of the GBU-43 Massive Ordnance Air Blast bomb is 0.011 kt, and that of the Oklahoma City bombing, using a truck-based fertilizer bomb, was 0.002 kt."