As enemy tanks grew in their inherent battlefield power and protection during the latter stages of World War 2 (1939-1945), warplanners went to work on all-new armor-defeating measures. There proved classic designs of the conflict such as the German "Panzerschreck" and "Panzerfaust" series while the British went into battle their "PIAT". The Americans found considerable success with their iconic "BAZOOKA" while work also continued on alternative weapon systems before the end.
The M18 became a product of this period. It was developed as a 57mm caliber static weapon with an inherent recoilless action to aid in accuracy. Its specialized design offered infantry a low-velocity Direct Line-of-Sight (DLoS) artillery weapon with good penetration capabilities at-range. It began life as something of a side program while development continued on a larger-caliber 105mm recoilless cannon influenced by the German 105mm Leichtgeschutz 40 infantry weapon (detailed elsewhere on this site). The new 57m crew-served American weapon - designated as the "T15" - was readied and in testing before the end of 1943. Because the T15 had such a great showing, work on the 105mm recoilless gun design was stopped and the new recoilless gun was adopted during 1944 as the "M18". Production ramped up in early-1945.
Design of the M18 was attributed to two men whose surnames, "Kroger" and "Musser", was used in name the weapon when it was in development (as the "Kromuskit").
The M18 worked optimally through a crew of two - one acting as the gunner/operator and the other as the dedicated loader/support personnel. Ammunition handlers would be part of the weapon's complete function as well. It was typically fired from a standard M1917(A1) machine gun tripod but operators could also find other, perhaps more desperate, means by which they could bring the M18 to bear - even firing it from the shoulder as is the case with more modern anti-tank weapons. Transporting the 44.5lb system typically required the services of both men and the 61.6 inch overall length also did not help things.
The action was aided by an interrupted lug rotating breechblock for loading/reloading. The mounting hardware gave an elevation span of +65 to -27 degrees so there was some tactical flexibility for the operators. Traversal was a full 360-degrees. The outgoing projectiles exited the launch tube at 1,200 feet-per-second out to an effective range of 500 yards and could reach targets, at reduced accuracy, out to 4,340 yards. Sighting was through the M86C 2.8-power telescopic sighting device.
The projectile, influenced by a British approach, utilized a rocket-like shape with short nosecone at front and a perforated cartridge case at rear. A pre-engraved rotating band was set nearer to the nose (to impart a rifling action during launch of the projectile) with a positioning band set near the midway point of the ammunition's length. The waterproof ammunition stocks were kept separate from the weapon until needed and was transported in a four-round, 40lb wooden crate. The gunnery crew typically had, on hand, about three to four ready-to-fire projectiles.
Engineers developed four distinct 57x303mmR rounds for the M18 (all weighing about 5.3lb): a standard High-Explosive (HE) round for dislodging enemy infantry and disabling light vehicles (the M306), a HEAT (High-Explosive, Anti-Tank) round for engaging enemy armor (the M307), a traditional smoke-generating round (the M308), and a basic training round.
Its late entry into the war meant that the M18 did not arrive in numbers needed to make an outright impact in the European Theater which ended in May of 1945. Fewer than 100 are believed to have been received before the German surrender though a stock was also sent Westward to the Pacific Theater. First-actions of the weapon, however, were recorded by American troops fighting in Germany where the armor-defeating properties of the M18 proved less than desired against the stout German Panzers - able to defeat only about 76mm of armor protection. The M18 gave better results through its HE round, especially by forward-operating elements, in the island-hopping campaigns of the Pacific where the enemy proved more static and dug-in for the long haul - while there was also a reduced chance of running into enemy tanks. First-actions in this theater were recorded at the Battle of Okinawa.
The M18 remained in service long enough to take part in the upcoming Korean War (1950-1953) and the HE round, once again, gave good service against stubborn, dug-in enemy forces. The HEAT round was still not up to the task of defeating the stout armor of enemy T-34 Medium Tanks so this task was left to the trusty "BAZOOKA" family of Anti-Tank weapons - its rocket capable of defeating up to 100mm of steel protection. Some M18s even managed to see extended combat service lives in the Vietnam War (1955-1975) where it was used as an anti-infantry measure by both the Army of the Republic of Vietnam as well as the United States Navy (fitted to assault craft in the latter for engaging onshore enemy positions). M18s were also used by the French in their fighting across Indochina and Algeria and may still appear on modern battles today from time-to-time - such was its proliferation.
Chinese factories copied the M18 outright, with slight changes to suit local requirements, and rebranded it as the "Type 36".