With the Mosin rifle on your back, specifically on the left side (not sure if that can change) moving forward (W) + right (D) causes the gun to clip through the first person view, it is especially bad if a bipod is attached to the rifle.

Addendum: Occurs when another weapon, such as pistol, axe, or flashlight, is held in the 'ready' position, but not when down. Must also be in crouched position.