The Swedish Pansarvärnsgevär fm/42 made by the Carl Gustav company was an interesting early hybrid antitank weapon - a recoilless rifle firing solid armor-piercing projectiles. It used a 20x180mm case, propelling the 108g (1650gr) bullet at 950 m/s (3150 fps). This was capable of perforating 40mm of perpendicular armor plate at 100m (a high explosive projectile was also made). This was on the high end of armor penetration for anti-tank rifles, and the m/42 was able to do this with a weapon weighing just 11.7kg (25 lb) - less than a quarter of a comparable 20mm conventional rifle.
This was possible because of its recoilless design - upon firing, the rear end of the cartridge case would blow out and vent out the back of the weapon, instead of being firmly sealed like a conventional rifle. This created a counter balancing recoil impulse which prevented the gun and shooter from having to absorb the full recoil energy produced by a heavy bullet launching off at high velocity. The tradeoff was that much of the potential energy of firing was wasted venting out the back instead of pushing the bullet forward, which is why the cartridge case was so oversized.
About a thousand of the guns were made by the end of World War 2, at which time even it had been made quite thoroughly obsolete by the rapidly increasing thickness of tank armor. It would, however, be the stepping-stone to the m/48 Carl Gustav 84mm recoilless rifle, which used a shaped charge warhead to perforate armor with a stream of molten melt instead of relying on velocity of a hardened projectile.
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