Okeh, so i'm blasting way out of Navarro. Eventually i realize something is different - i no longer hear the sound of Cassidy's Pancor Jackhammer. When the smoke clears, i see he is now using a Pulse Pistol. The Jackhammer is in his inventory and i have no idea where the pistol came from. What's more, he seems to like it, as he stays with it after clicking 'Use Best Weapon'. He didn't change until i gave him a Gauss Rifle. I can theorize that he stopped using the Jackhammer after a critical failure, except it was still fully loaded.