The British were the first to invent the tank during WWI, and so the Germans were forced into the position of being the first to undertake the difficult task of developing anti-tank weapons. The Kaiser's army approached the issue of protecting infantry from tanks in a pretty straightforward way: if regular machine guns and rifles can't penetrate the new British wonder weapon, then make a larger rifle.
The first rifles against the first tanks
The idea of building a larger caliber weapon led to the Tank und Flieger (TuF) Maschinengewehr machine gun and the Tankgewehr (or T-gewehr) rifle. Both were designed to use the new 13,25×92SR round developed by the Polte company. The machine gun was essentially an enlarged MG-08 (the German variant of the Maxim gun). The Germans didn't have time to properly ramp up production of the machine gun before they surrendered, but they got the chance to put the first ever anti-tank rifle to use. Single shot variants of the Mauser rifle made it to the front lines and even claimed some enemy tanks, according to some sources.
The MG 18 TuF (Tank und Flieger Maschinengewehr) heavy machine gun.
The Treaty of Versailles radically restricted the Germans' ability to develop new weapons. In any case, there was little desire to develop the heavy machine gun any further. Maxim's descendant ended up being too complex and expensive, and at a mass of 130 kg it was easier to just make a cannon. However, the Tankgewehr was quite successful. It was used in the Reichswehr and then even in the Wehrmacht for training and various experiments.
Other participants in WWI didn't ignore the German experience. The USA began development of an anti-tank and anti-air machine gun on the initiative of the commander of the expeditionary force in Europe, General John Pershing. The Americans didn't try to reinvent the wheel and enlarged the .30-06 rifle cartridge. A comparison of their design with the German one obtained after capitulation led them to believe that their round was more successful. This was a correct evaluation, as the 12.7x99 mm or .50 BMG round remains the standard large caliber NATO round and one of the most popular rounds in the world. While it was not often used as an anti-tank weapon, the .50 cal remains potent against lightly armoured targets, especially when using AP bullets.
The best known photo of the Tankgewehr was taken by the enemy. In this picture, New Zealand infantrymen pose with a captured anti-tank rifle. August 1918.
The USSR built their own analogue of the Tankgewehr in 1938, but using the Soviet 12.7x108 mm round. Initially this was done only as a baseline for testing new Soviet anti-tank rifles, but with the start of the Great Patriotic War it suddenly turned out that the Red Army was not exactly saturated with anti-tank weapons. One of the stop-gap solutions was to put the "Soviet Tankgewehr" into production. Post-war literature often calls this weapon the "Sholohov anti-tank rifle", but this designation is not used in period documents. As a rule, it was called "12.7 mm Mauser type anti-tank rifle".
Playing with geometry
Let us return to interbellum Germany. The Germans were performing experiments with their remaining Tankgewehrs, but had no desire to needlessly antagonize foreign observers. Work on anti-tank rifles was transferred to the Swiss Solothurn company, which was incidentally purchased by Rheinmetall in 1929. The letter of the law was followed: weapons forbidden to the Germans were now being developed by the peaceful Swiss rather than the warlike Huns.
.280 Halger round used by Herman Gerlich in his rifle.
Another potential solution was developed in the Weimar Republic even without the intervention of the military. Herman Gerlich, the chief engineer at Halger, a small hunting and sporting arms company, was obsessed with the idea of creating a hypervelocity bullet. This gave a great advantage to hunters, as they could do away with a large amount of mathematics involved in long range shooting: range estimation, wind correction, etc. The effect of hypervelocity bullets on living tissue, especially if they struck bone, was devastating.
In 1929 Gerlich presented his new rifle chambered in the .280 Halger round. He humbly called it "the fastest and deadliest gun ever made". Initially, specialists were skeptical of his claims, as they seemed quite fantastical. However, further trials confirmed the engineer's claims as well as revealing that his "secret ingredient" wasn't that new after all. In traditional systems some amount of gases overtake the bullet, which can be seen well with modern slow motion video. Gerlich used a conical barrel that narrowed towards the muzzle, which used the expanding gases more efficiently. The idea of using such bullets and barrels was first proposed by German professor Karl Puff in the early 20th century.
Gerlich developed Puff's idea further and managed to build a working rifle with a conical barrel. The German military showed little interest in the idea, but this was hardly a disappointment for Gerlich, as his rifle was being trialed in the UK and USA starting in 1929. A muzzle velocity of 3500 fps (1066 m/s) was promised for the hunting rifles. Indeed, trials held in the UK in March of 1931 showed an average muzzle velocity that was even higher than claimed. Some bullets reached 3812 fps (1162 m/s). According to some sources, a muzzle velocity of 4400 fps (1341 m/s) was recorded during American trials held at Aberdeen. Some sources also state that Gerlich managed to reach a muzzle velocity of 1400 m/s with his 7 mm rifle, with the ability to increase it further to 1600 or even 1700 m/s.
A surviving Gerlich rifle with a conical barrel.
It quickly turned out that the bullets were not just effective for entertainment. Armour plate behaved atypically when struck at velocities of over 1150 m/s. The bullet didn't just penetrate the armour, but shattered it like glass.
Herman Gerlich died in 1934, according to some sources of not exactly natural causes. By that point his work took on a life of its own on both sides of the Atlantic. For instance, Larsen, a Dutch company that Gerlich was planning on working with to produce his rifle, offered a 29/20 mm gun to France in 1940 to replace their 25 mm anti-tank guns.
Simple, but not best
Initially, the bargain hunting Germans aimed to avoid conical barrels, as the luxury of spending valuable high quality steel on barrels with a lifespan of just a few hundred shots seemed out of reach. Rheinmetall developed a 7.92 mm round (a traditional caliber for German small arms) with an enlarged case. With this round, engineer B. Brauer developed a new anti-tank rifle called Panzerbüchse 1938 (Pz.В. 38).
7.92x94 mm round used in Pz.B. 38 and Pz.B. 39 anti-tank rifles.
This was a single-shot rifle with a semiautomatic vertical sliding breech. A spring braced against the breech acted as a recoil compensator. The first rifles were equipped with drum containers holding 36 rounds, later they were replaced with simpler boxed holding 10 rounds each attached to both sides of the gun.
A muzzle velocity of 1200 m/s gave the Pz.B. 38 30 mm of penetration at 100 meters. This was enough to successfully combat tanks with bulletproof armour. If the shooter was lucky, he could even penetrate the side of a more heavily armoured vehicle.
The Germans were rather skeptical about the ability of an anti-tank rifle bullet to deal significant damage to the vehicle or its crew after penetrating its armour. An attempt was made to insert a tear gas capsule into the bullet. It wasn't much, but when added to fuel and gun fumes it could be enough to drive the crew out of their tank.
Pz.B. 39 rifle with ammunition containers attached to the sides.
The combat debut of the Pz.B. 38 in Poland was not entirely successful. Sure, it could penetrate the armour of Polish tanks, but the users complained about the excessive mass and size of the Pz.B. 38 as well as reliability issues, especially in dirt. Brauer was forced to rework his gun, simplifying it and also improving reliability and reducing size. The new rifle was called Pz.B. 39. This was the anti-tank rifle that German forces were using when they crossed the Soviet border on June 22nd, 1941.
By July 7th one Pz.B. 39 was already at a Soviet proving grounds near Moscow, although only 7 rounds of ammunition were captured along with it. This was enough for a trial. The rifle penetrated 30 mm of armour at 30 degrees from 50 meters and 40 mm of armour at normal. This meant that the rifle was dangerous to even new Soviet tanks at a range of up to 100 meters.
One of the best known photos of the Panzerbuchse: a German anti-tank rifle crew with a Pz.B. 39, summer 1941.
As mentioned above, the USSR never managed to bring any of its many experimental anti-tank rifle to production, and so a decision was made to copy the German weapon in parallel with development of a domestic anti-tank rifle. The Tula factory proved capable of copying the rifle, but not its ammunition. If the German rounds developed a pressure of 2600-2800 kg/m², Soviet ammunition needed over 3000 kg/cm² to develop the same muzzle velocity. This kind of overload reflected negatively on the reliability and lifespan of the rifle. The following conclusions were made after trials held in early October of 1941.
"The penetration obtained with domestic 7.92 mm metal-ceramic ammunition, even with a new rifle, does not fully enable combat against enemy light tanks which have 30 mm of front armour. After 100 shots the rifle becomes useless against armoured targets."
The GAU concluded that the project should be closed, although a number of rifles that were already produced were used in the defense of Tula.
Soviet anti-tank riflemen with Degtyaryev rifles.
The USSR later developed an analogue of the German Geschoss 318 bullet with a chemical filler, but it was never mass produced. It's unlikely that the Soviet government was afraid of accusations of chemical warfare, but there was already enough trouble with 14.5 mm ammunition production.