strange shift of an 3d model

Hey

Lets try to describe my situation.

I've a player with a weapon.
So when the player watch in the sky this happened with the weapon:

if the player watch forward this happened:

and if the player watch down this happened with the weapon:

why it is so and how I can fix it.

Sry. my english is very bad -.-

But I hope you know what I mean.

Have a nice day

Moritz