.50 AE, and .50 BMG are two different animals. Yes, both can be supressed. But for the gun and round to be truely supressed, a sub-sonic round must be used. For the HAR-15A1, this isn't a bad thing per se, as the weapon with sub-sonic ammo would be fantastic for sentry removal. Slap in a mag with high-velocty ammo, and you've got a decent counter-sniper weapon as well. Obviously the weapon would have to be significantly modified, and accurized for this to work reliably.
But for the .50 BMG, in this context the weapon would be used in the anti-material and heavy-sniping role. High velocity is key here, to pentrate lightly-armoured vehicles, and deliver maximum energy to defeat targets.
Still, I've seen quite a few .50 BMG weapons equipped with large supressors/flash hiders/muzzel brakes. The weapon's firing location is still relatively well-masked when these are used, but the supersonic "crack" of the round would at least alert any survivors of the first shot, that subsequent rounds might be on their way...