It was announced in October 1983 that FN were seeking to develop a more powerful replacement for the .50 BMG, better able to penetrate light armoured vehicles (in effect, a Western equivalent to the Soviet 14.5mm KPV heavy machine gun).
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.
[1] Spent shell casings were ejected out of the bottom of the gun, as in FN's later P90 personal defense weapon.
The project was cancelled in the early 1990s, as FN shifted its focus to the P90 personal defense weapon.