The BRG-15 is a gas-operated weapon using a rotating bolt, and had a cyclic rate of 600 rounds per minute. However, its most distinctive design element was a dual-feed system (in the vein of the HAFDASA C-4 submachine gun), in which ammo belts feed into the weapon from either side. The operator can switch between any of the belts by using a selector. Spent casings eject from the bottomside of the weapon, in a manner similar to FN's own P90 PDW.
It also features a quick-change barrel, and had a tripod system developed for infantry usage. It was likely, however, that the BRG-15 would have been vehicle-mounted, where the dual-feed system could be put to best use.
The BRG-15 uses a very large caliber 15mm (later 15.5mm) round, of which several variants were made. Early development of the ammunition used a 20×110mm Hispano casing necked down to accept a 15mm projectile. However, the projectile size lead to rapid barrel wear. The result was the development of a 15.5mm projectile with a plastic driving band.
The caliber began as 15×110mm, and then transitioned to 15×115mm with an increased casing length. The most recent caliber used for the BRG-15, at 15.5×106mm, was FN's second attempt at competing with the KPV heavy machine gun's 14.5×114mm round, appropriately by necking out the round's case to accommodate the 15.5mm projectile.
It was first announced in October 1983. The BRG-15 was developed as a more powerful replacement for the ageing Browning M2HB heavy machine gun. The project was reportedly canceled in favor of the P90 PDW, due to FN Herstal having financial difficulties at the time.