How was this animation made? I mean, how this weapon was attached? If you look closer, the character slides his hands over the weapon, his hands are moving, changing its positions by the weapon to make the attack (you can skip to 7 seconds of video to see better). How this is done? Was it sockets? but with sockets, the weapon shouldn’t follow his hands (and would give a very buggy result with this slide)? What is happening here? This threw me a curve ball, really… Anyone who knows to answer i’ll be very thankful.

Assuming the Epic rig I would say the the weapon is part of the character and is weighted to ik_gun. From there easy enoguh to use a parent/child constraint to switch from left to right hand use.

Yeah, looking at the video, I also think the weapon is part of the skeleton.

You can look at paragon characters for a hint. All of their weapons are built right into the character’s skeleton. The upside is you can create very complex animations and the downside is that the weapons can not be changed as easily as separate objects attached to sockets.