: The side-mounted magazine is probably on the other side of the gun as the
: charging handle is on the left on the new model. Alternately, the magazine
: might be in the pistol grip. The bottom of it where it's a lighter grey
: looks like it could be a magazine that contains handgun-caliber rounds
: (which by definition is the type of ammo an SMG uses).

Putting the magazine on the right side of the weapon would make for a very awkward mag change for right handed shooters. That does look like a magazine floor plate on the bottom of the grip, but it doesn't align well with the receiver since the grip has that angle change near the top. I can't picture it.

And the Halo SMG uses caseless ammunition, if I remember correctly.