 FN initially selected the 20mm Hispano case as a basis and necked it down to 15mm for the FN-BRG (15x115).The weapon and its ammunition had a protracted development and finally emerged as a 15.5mm because of fast barrel wear.The 15.5x106 was FN's second effort to make a KPV competitor in the BRG-15, appropriately by necking out the KPV's case.