15.5×106mm round. BRG-15?
I bought a small ammo collection some years ago, and found this interesting piece between the rest of all the rounds.
According to the documentation of the previous owner, it belonged to an experimental heavy machine gun made by FN called the BRG-15. But it was shelved in favor of the P90.
I know most people here know more about WWI - WWII ordnance, but where else could i ask? There are no markings on the base. .50 is for size comparison, don’t mind the unstruck primer, its an empty round 
I would just like some confirmation and maybe if someone knows if these are rare or that many of these are in circulation?
Thanks!
