The PR at least would be much lighter because the ammo doesn't have casings and it wouldn't need all the gas operated functionality to operate it.
The smartgun is never going to look right because in reality it is a non functional weapon. You need to be able to hide behind some kind of cover in a firefight or otherwise your just a big standing target. The smartun, despite what the technical manual says, was obviously meant to be fired from the standing position. And I understand it has lots of firepower, but having the smartun doesn't equip the user with deadeye. All that incoming fire is going to be directed at the smartun user. It is a cool looking, but silly and non functional design for anybody not wearing call of duty juggernaut armor.
The fact it looks weightless bothers me the least about it.
But again, you won't have natural movement because in third person games you cross hair is where your character is looking, and the positioning of the body focuses on that. Only when you center your body to aim does everything start looking more natural and the weapon system should be closer to the body.