Despite its bullpup configuration, the weapon was still bulky and cumbersome to handle on the run or on march. It weighed an obscene amount though its battlefield benefits seemingly outweighed its tactical limitations - particularly in the desire to fire such a large cartridge (a design challenge even today). The cartridge itself was nothing more than that used by the S18-350 aircraft automatic cannon which serves to provide the reader with an idea on its size and original intended use/function.
Despite its Swiss origins, the S18-100 fell to use by the German Army of World War 2. Solothurn had been purchased by the German concern of Rheinmetall as an outlet to design, produce and sell war-making goods around the restrictions placed through the Treaty of Versailles that appeared after World War 1. In this way, the company could still exist doing what it did best and would end up arming German forces underneath the nose of the watching world - however, not all these guns ended up in German hands.
Following the Soviet invasion of Finland in November 1939 to begin the famous "Winter War" (Nov 1939 - March 1940), the Finnish Army obtained some S18-100 series guns by way of the Swiss Army but these arrived too late in the field to be of much use. They were pressed into action during the Continuation War (June 1941 - Sep - 1944), however, as the Finns were now aided by the Germans in their assault against Soviet forces. Despite their inherent power, the weapons did not prove of much value in the extended fighting which led to the development of the higher-powered S18-1000 model chambered for the 20x138mmB (Long) cartridge. The Germans made use of this form as the "PzB 41". The S-18/1100 then followed with an included automatic action and 20x138mm cartridge but many of the inherent limitations of the original design still remained (weight, length, recoil force, expensive).
Beyond its use by Germany, Switzerland and Finland, the weapon saw service with the forces of Bulgaria, Hungary, Italy, Mexico and the Netherlands before its history was truly written. Indeed the weapon was recorded used in the short-lived Slovak-Hungarian War of March 1939 - April 1939 as Axis-aligned Hungarian forces successfully invaded neighboring Slovakian territory of the First Slovak Republic. By the end of the war in 1945, better systems were in play including the famous American M1 "Bazooka" and German "Panzerschreck" lines which were adopted in large numbers. The Soviets held a particular regard for their anti-tank rifles and elected to remain using them over rocket launchers in their fight against Germany - an interesting decision considering they originated the famous RPG (Rocket-Propoelled Grenade) line of the Cold War decades.
It should be noted that, for its time, the Solothurn S18-100 line was a stout performer against armor of the period. It was simply done in by the advancing nature of tank warfare and the rise of shoulder-fired, armor-defeating rockets coupled with its limitations and expensive/complicated manufacture.
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