The FN BRG-15 was a heavy machine gun project made by FN Herstal, it was abandoned in the 1990s to instead fund the project of the P90.
However, the FN BRG-15 is the origin of the quick barrel change that was later adapted to the M2HB and the bottom round ejection that was later used for the p90.

This gun is absolutely kickass and i really think it should have seen the light of day, at least past the testing range.

I got most of my information about this from https://www.historicalfirearms.info/post/157250176539/fn-brg-15-in-the-early-1980s-fabrique and https://en.wikipedia.org/wiki/FN_BRG-15