High-Explosive-Incendiary-Armour-Piercing rounds are meant to be used by Snipers for destroying material, light vehicles or other "opportunity targets", like a weak spot on a tank.
The Connor Family used Raufoss Mk 211 rounds to destroy Terminators, but this is not something that snipers do in real-life., and Terminators are not humans.
Of course these rounds can be used against humans, although Laws of War might not allow it in real-life, but Snipers are meant to take one shot, maybe two, and then get out or change location, so they are not going to waste their ammo on "low-value" targets.
Demolitionmen on the other hand, use explosives to blow up stuff, and it makes sense if they can fire Hi-Ex rounds out of their M4s, because their weapons are not specialized Sniper weapons, and blowing stuff to smithereens is their business.
The M99 firing Raufoss Mk 211 is something we made up, in this thread i think, to justify the ammo damage. In-game, there is no blowing up with the M99.