The Solothurn S-18 is a Swiss anti-tank rifle.
Solothurn, owned by Rheinmetall, was used by its parent company to produce weapons that were prohibited to be produced by German manufacturers at the time. Thus, the S-18 was born.
The USA proposed to use the S-18 in their army in 1940, but after comparison to a .90 cal T4 gun, they found that the Solothurn performed a little worse, and was much more complex. The plan was to produce 50 pieces and later get the actual gun produced in the US, but deals never worked out.
The S-18 was recoil operated and fired a 20mm cartridge. The gun had a manual extractor to help eject the casings manually. It also had a recoilling barrel.
The gun was very complex and in no ways easily portable; carrying it requires more than one man. Due to its sheer weight, it cannot be carried for long distances. The cartridge also made recoil tremendous.
This is the smaller rifle, firing a 20x105mmB cartridge. Despite being the smallest, it was still rather heavy and needed to be lugged around.
This is the bigger rifle, firing a longer cartridge. It can be identified from a S-18/100 by a crank on the right hand side of the rifle. As with the S-18/100, it is in no ways easily portable.
This is basically an S-18/1000 with the capability for full auto fire.
This is an unlicensed copy of the Solothurn produced in Estonia.