Ah, yes. There was that. The Doctor held the only key to the TARDIS, now, and any thought that he should recreate one for Jack was eliminated the moment he told him he was no longer welcome on the TARDIS. Wasn't it?

The Doctor reached down and pressed his hand against his aching wound. Being a companion was about trust. The Doctor couldn't trust Jack.

"Yeah, suppose so."

Well, he could trust him with his life and with his TARDIS, but not with a gun. Maybe if guns were just eliminated from the picture completely, things would be different.