Thanks for your reply SilentW, I already looked into the code but really I dont understand how UE4 is making the hierachy of classes, I will need to take a hard look.
About the hands, I saw in the editor that they are attached with one single bone, the right hand is attached to the root bone of the weapon, or atleast the example weapons doesnt have any other bones either than the magazine bone, I tryed moving it but it does not affects to the left hand possition.
Finally I figured out a way, but its not the best, by simply making first person arms invisible and spawning a weapon with arms included, sure I need to animate them like a traditional FPS game, but this way I will have especial arms/animations/ for each weapon.
About the weapon rotation, with the method I figured out I should be able to control it trought 3Dmax, but its a bit annoying because it needs of tryal and error. (I didnt found any param on the skelmesh viewer to modify the default location/rotation)
If someone finds a better way I woula like to know about.
---- Update ----
Here is a video of the result of my solution: