I am sorry if this is a duplicate.
When you punch with your sword or move your mouse very quick form the top of the screen to the bottom, you will see that the tool floats.
MC-76678
No hand model for tools
Resolved
Works As Intended
0
marcono1234
Unconfirmed
arm, hand, model, tool
Minecraft 1.8.2-pre4, Minecraft 15w37a
Retrieved
I don't think mojang intended to have a model for the arm when you are holding something in your hand and are looking in first person mode.
So: Intended behaviour