From the Internet:The ammunition is a NATO standard 35 mm x 228 mm. It’s inconceivable that the Swiss would be the only manufacturer of ammunition. This is rather hypocritical of the Swiss anyway. They had no problems providing that ammo to other countries who fight wars.
The 35x228 Oerlikon KD series was developed in the 1960s to replace the Swiss 34mm Flab Kan 38 AA gun. The AA gun has been exported to many different countries and remains in wide service today, including being produced in China. It is always found in twin mountings, towed, mounted on vehicles or fitted to turrets (the best known being the German Flakpanzer Gepard). In addition, a slower-firing long-recoil Oerlikon KDE using the same ammunition was developed for the ground-firing role in light AFVs; this has only been adopted by Japan. The most recent Oerlikon gun development is the KDG revolver cannon which fires at no less than 1,000 rpm. This is offered in single mountings for both land and naval use.
The ammunition has developed too, the latest type being the KETF (Kinetic Energy Time Fuzed), formerly known as AHEAD, which is an advanced shrapnel type. Different versions of this have been developed for AA/anti-missile and anti-personnel roles.
During the 1980s Rheinmetall started developing the Rh 503, an externally-powered weapon available with two barrels; one to take the standard 35x228 ammo, the other to take a 50x330 "Supershot" round. The reason for the 50mm is to provide the biggest possible "piston area" for firing saboted rounds, particularly the APFSDS, with the bonus of firing a bigger HE shell. It was intended for the next generation of German AFVs (starting with the 35mm caliber, with the 50mm available as a later upgrade if required) but the end of the Cold War put a stop to it, Germany having selected the cheaper option of a 30mm Mauser cannon for their new light AFVs.
The overall length of the two cartridges is the same; only a small part of the sabot protrudes above the neck of the 50mm case, so little more than a barrel change is required.
ATK (or to be precise one of their earlier incarnations - anyone who can keep track of the endlessly shifting paternity of military weapons deserves a medal) has developed a version of the Chain Gun family - the Bushmaster III - to use the same 35x228 ammunition. This has been adopted by Denmark and the Netherlands in their CV9035 MICVs.