This is because of the Metalic properties of the model that also make them slightly bullet-proof and cause Slow Motion falling. Models aren't used to having solid properties on them, and it causes the engine to treat the models in a strange manner and causes the said issue. This also occurs with with the Nailgunner/Replica Heavy armor in my FEAR 2 NPC addon. Nothing can be done to fix this without removing the metal properties from the models.