Throughout the Cold War, the Soviet Union served as the potential enemy to Europe and the US military, in large part, was charged with the mission to counter the expected tank and infantry invasion to come across the divided German border. As a result, the United States Army was always interested in the largest, most powerful weapon on the battlefield and, at that time, this proved to be a tactical nuclear weapon. The US had three atomic cannons in inventory - the "Atomic Annie's" that were largely hidden and relocated as needed, their crews waiting for the order to fire. The size of these Annie units, their required tow vehicles and the numbers of men made them relatively difficult to realistically conceal - thusly easy to track and observe for Soviet elements. If the Soviets decided to invade Europe the Annie's would surely have been one of the first targets engaged. Additionally, the only other nuclear delivery weapons at the time were expensive bombers.
In the general military mindset of the era, it was deemed a requirement for more covert, squad-level nuclear weaponry for the US Army. In 1956 a small device was developed for a highly-trained infantry unit called the "Atomic Battle Group" (ABG) to be used for policing the border between East and West Germany. Called the "Davy Crockett", it was a 155mm caliber tactical nuclear recoilless gun in the standard M28 or longer-range M29 launch body firing the M388 nuclear projectile. The launcher piece was developed at the Rock Island Arsenal and the W54 warhead was developed at Los Alamos and the Atomic Energy Commission. The designers chose the name Davy Crockett after the American folk hero, woodsman and congressman who told the story that, being out of ammunition one day and coming upon a bear, he had to "grin the bear to death". The Soviet national animal was a bear so it seemed fitting for the men in Atomic Battle Group waiting for the Soviet "bear" to attack. Some confusion existed that the M-388 was a "recoilless rifle" meaning that the tube/barrel would have helical grooves forcing the round to spin as it was propelled out of the barrel. This "rifling" was used in rifles, pistols and even barrels as large as the massive Krupp K5 for accuracy at range. However, the M-388 has designed with a smooth heavy tube featuring no rifling - known as "smoothbore".
Between 1961 and 1971 the Atomic Battle Group's mission was the protection of Europe though fewer than 100 Davy Crockett weapons were ever deployed. The purpose of the Crockett was to stop the advance of enemy troops and tanks by the sheer power of its nuclear explosion, ultimately and immediately dispersing the effects of radiation at the explosion site. If this deterrent was necessary, the US Army calculated it would stop the enemy and give them 48 hours to counter the Soviet invasion with a conventional response.
Operation of the Davy Crockett was more or less conventional. First the propellant charge, wrapped in a thick paper case, was inserted into the launcher barrel from the muzzle end. Next, a hollow metal cover was dropped in to cover the propellant. The end of the warhead was fitted into the top opening of the gun launcher allowing the warhead to extend outside of the tube opening somewhat. The metal case surrounding the propellant charge was perforated with dozens of holes to allow the propellant gases to escape when firing. Range was accomplished by specific selection of the propellant charge. The "recoilless" classification of the weapon simply implied that the gun did not feature the inherently violent backwards travel when fired as common to most any other gun (though some amount of recoil was present to be sure). The appropriate classification for the Davy Crockett 155mm system therefore was recoilless "launcher", "projector" or "gun".
Two distinct calibers of the gun were ultimately built: a smaller 120mm (XM-28/M-28) model with a range of 1.24 miles and a larger 155mm (XM-29/M-29) unit having a range of 2.49 miles. Both launchers fired the same XM-388 nuclear projectile. The warhead projectile XM-388 was bomb-shaped in the traditional sense with a round windshield nosecone and four stabilization fins along the back end. The entire projectile weighed 76 pounds and was 2 feet, 6 inches long with a diameter of 11 inches. The W54 warhead portion encased in the projectile weighed 51 pounds. The weapon had an automatic lethal radiation exposure radius of 10,000 rem out to164 yards (150m) at the blast site and a delayed fatal dose of 600 rem within 150 yards (137.16m) of the blast center - of course this was all dependent on wind direction. The M-388 three-man crew would be exposed with this lethal dose - in effect making them expendable assets. A typical Davy Crockett detachment consisted of three personnel.
The original Davy Crockett concept envisioned an infantry squad carrying the launcher, tripod mount and warhead into battle and setting the launcher upon the freestanding mount, loading the weapon and engaging enemy units. Later, the concept was further developed to provide mounting the system on jeeps or tracked vehicles and this allowed the carrying of multiple warheads.
When the crew was ready, an M101 "spotting" round would be fired from a small 37mm rifle attached underneath the Crockett launch tube (the rifle was added as standard issue only later). Depending on the spotting round's landing, the officer in charge figured the angle and timing calculations verified to within 100 feet of the target. Based on the advancing force positions, the officer would decide the height of burst by flipping a switch on the side of the warhead. The firing crew would also receive a significant radiation dose within the 1.7 mile blast radius so it was encouraged to fire the weapon from behind a hill and keep a head down during detonation. If a position overrun seemed likely, some reports suggest that the unit in charge of the Davy Crockett was charged with destroying their system - such was the secrecy of the technology at the time.
When fired, the Davy Crockett M-388 would soar from the launcher at 100 mph with an appropriately loud "bang" and produce a heavy cloud of white smoke arcing out towards the target. Once fired, the nuclear device could not be aborted en route. The 37mm spotter gun had a rifled barrel but the simple launcher's smoothbore design made for poor accuracy at range. Test shots were often hundreds of feet off the target mark. However, even a poorly located atomic blast was still deadly so the fault were negligible. The blast damage would be great and the radiation would make up for the inaccurate trajectory. The kill zone was 1/4 mile from the blast site within days or weeks and anyone with 500 feet would be dead within hours.
In 1963 American President John F. Kennedy was reviewing the 3rd Armored Division at Hanau, West Germany and spotted the Davy Crockett on display, stopping to talk to the soldiers nearby. Kennedy asked about the accuracy of the weapon and was told of its destructive power. Later that year President Kennedy ordered the removal of all Davy Crockett's from Germany though they did remain in the United States Army inventory until 1971. In its place, nuclear-tipped artillery shells began deliveries in increasing numbers.