Temporal grace

Temporal grace was a term used to describe the state in which the interior of TARDISes existed. Supposedly, it ensured that no weapons could be used inside its environs. (DW: The Hand of Fear, The Invasion of Time) However, on other occasions, it seemed that no such defense existed. Cybermen (DW: Earthshock, Attack of the Cybermen) and a Dalek, for example, have discharged their guns in the TARDIS. (DW: The Parting of the Ways) Nyssa presumed the grace broken in regards to the Cybermen having been able to fire weapons inside the TARDIS. The Doctor simply replied "nobody's perfect". (DW: Arc of Infinity)

Behind the scenes

 * It is possible that the Doctor is able to disable temporal grace, or that it is merely a bluff to convince people not to attempt violence in the first place. If the Doctor can disable it then a possible explanation for why it was deactivated in Attack of the Cybermen is that he was using the sonic lance to make repairs. Since this device was used as a weapon it may have been perceived by the TARDIS as one and this prevented him from using it.