Refer to updated T142225
(most apparent with the AKM) if you happen to jump in certain directions, the IK poses for the gun you are holding completely breaks.
Also please disable jumping when in ADS or raising guns, or else it leads to even worse IK Pose issues!!
Another way weapon IK poses can be distorted through online synchronization is when you switch weapons on the hotbar very fast resulting in the slung weapon in a diagonal weird pose, or the weapon you're holding not sync with your hands.
(This I'm very unsure of, and can completely be the wrong way to reproduce)