Jack had nearly lost his mind when he'd returned and seen John missing. John had left and that could never, ever be a good thing. Particularly when he had no idea what was available for him. Luckily, he didn't have to wait long to find out.

"John," Jack's voice was full of fire. He was threatening the Doctor. His Doctor. guns... No guns with the Doctor. He'd removed his pistol back at the Hub in effort to keep John's hands off of it, and now he was sincerely regretting that decision. "Put. The Gun. Down. NOW." There was always the possibility that John might pull the trigger. He sincerely hoped it wouldn't come to that.