The 4,3×45 and the 4,6×36 cartridges were steps in the development of the G11 rifle and the caseless ammunition of Heckler & Koch. Early trials used the 4,9mm caliber. By February 1973 a decision was made in favour of the 4,3mm caliber because of its better external ballistics. The .17 Remington sporting cartridge served as a reference.
The extensive pilot tests to optimize bullet form and inner barrel profile did not require the use of expensive caseless ammunition, therefore the 4,3x45mm cartridge was created. Its case was a modified 5,56×45 and the weapon used for the tests was an altered HK33 rifle.
According to Peter Petrusic in the ECRA newsletter 381-13, this bottom specimen was a VELOCITY TEST. It has a CN Molybdenum coating. Apparently only 300 loaded