Hello,
I have this problem for many days now and unresolved.
The details are in this link.
http://answers.unity3d.com/questions/697142/how-to-properly-place-a-rifle-weapon-into-the-char.html
Thank you.
This is the best community ever.
If you found an answer to you question Id appreciate you sharing it with us, Im also exploring this area currently 
Thanks for the reply.
Unfortunately no. I asked this many weeks ago. No answers yet.
My progress of my game (which is due in 5 weeks) is halted because of this small problem.
This should be posted on support…
Make the gun a child of the bipe model’s hand root.
Moving this to support
Inverse Kinematics: Unity Asset Store - The Best Assets for Game Making
Based on the images you provided on that link, I’d say the problem is with your animation, not with Unity. What’s up with his fingers?
The animations is made by Unity (Raw Mocap Data and Bootcamp) so there is nothing wrong with it. It is suppose to be holding a M4 with a grenade launcher thats why his fingers are positioned that way.
It already is, I mentioned it on the post.
There are things wrong with his fingers, definitely. I’m not sure mine can bend upwards. 
Other than that, not sure what could be causing the problem.
There are only three reasons I can think of for this to happen: It’s not properly parented; it’s being moved by a script; or the animation is messed up. If it’s not the first two, it’s got to be the last and judging by that picture, it is most definitely messed up. The left hand is upside-down so the fingers and bending backwards! Chances are the right hand needs to be rotated to make him hold the rifle properly. Just because an animation plays doesn’t mean the positioning works for what you want.
I really don’t know what to do now. I just want my character to aim his rifle. Can anyone help me build me this from scratch?