The rise of the tank in World War 1 (1914-1918) forced the development of tank-killing systems to follow. Artillery proved a major factor in destroying tanks early on, as did land mines and even trenches played their role to an extent. Throughout the interwar years, tank development reached all new heights as the traditional form of a turreted armored tracked vehicle was accepted from the World War 1 French Renault FT-17. From this came the classic tanks that dotted the World War 2 (1939-1945) landscape, particularly in the European Theater and brought tank warfare into the forefront of modern combat.
The prominence of tanks forced engineers to develop man-portable, armor-defeating systems in turn. For a time, the Imperial German Army in World War 1 relied on a heavy, single-shot, bolt-action rifle as through the Mauser Model 1918 "T-Gewehr" - essentially an oversized rifle firing a large-caliber cartridge. Its arrival marked the start of the "anti-tank rifle" as a military weapon category and some 15,800 of the type were produced. While not as effective against tanks on the whole, such weapons could target key weak points in a given design, engaging driver/gunner positions or critical mechanical components in an attempt to disable the tank at range - a disabled tank on the field was a reduced threat when compared to a mobile one.
The idea of anti-tank warfare, heading into World War 2, remained largely unchanged save for the introduction of dedicated anti-tank mines and larger-caliber towed anti-tank cannons (beginning with the 37mm types). The anti-tank rifle eventually appeared in a few notable forms such as the British Boys Anti-Tank Rifle and a pair of Soviet designs. By this time, the Swiss had quietly developed their own in-house solution under the designation of Solothurn S18-100, a massive rifle system relying on a semi-automatic, recoil-operated action while firing the large 20x105mmB (20mm) cartridge from a 5- or 10-round box magazine (fitted into the left side of the receiver). The weapon measured 1,760mm long with a 925mm barrel and weighed some 45 kilograms (100lbs) WITHOUT the magazine installed.
The Solothurn rifle was born in the early part of the 1930s as engineers recovered a World War 1-era Erhardt 20mm cannon design of 1918. Improvements were made to the action and the weapon was placed into trials. Once ready, it was adopted in small numbers in 1934 by Switzerland, Italy and Hungary.
The arrangement of the S18-100 was rather unique for the period as it utilized a "bulpup" configuration in which the magazine and action were held aft of the pistol grip and trigger unit. This concentrated the bulk of the weight towards the rear and allowed a full-length barrel to be used in a more compact form (theoretically). The arrangement gave the S18-100 a bulky appearance nonetheless with a tubular forward shroud found at the base of the barrel and the barrel extending a distance ahead of the shroud. The barrel was capped by a perforated muzzle brake to help contend with the massive recoil effects of firing such a large and powerful cartridge. The stock was padded for resting against the shoulder while a hinged bipod assembly supported the frontal section of the weapon and hinged bipod was added to the rear, under the stock. Optics were set along the left side of the weapon with the ejection port affixed to the right. Taken as a whole, the S18-100 appeared as a very forward-thinking design, its lines akin to the systems encountered on the battlefield today.
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.