The magazine placement. In order to reload, you have to lift the rifle. It'd make more sense if the magazine stuck out to the side. Better yet, you could make the rifle shorter because the magazine can be mounted above the pistol grip, rather than in front.
In fact, that was actually a selling point of a real-life anti-materiel rifle. (More military hardware should get commercials. Seriously.)

I understand that this tends to unbalance the weapon a little bit, so the magazine could be mounted on top of the gun, and the sights could be off to the side. It'd be a unique weapon in first-person shooters, I promise you that. ; )

Also, someone finally brought a feature of MJOLNIR to life: Magnetic clamps that hold your weapons and ammo.

I wouldn't trust it as a holster, but it's pretty neat.