AD Abbrev: Accidental Discharge
For it to be an AD the firearm must have a fault, which is rare. Soldiers that have AD's on the range usually carry on shooting for the rest of the range day without a problem, this in itself proves that the weapon is working properly and thus your AD is an ND. If you have an AD unload the weapon put it down and ask for an Armourer (Brave, intelligent and handsome REME tradesmen) to inspect it for faults.... you might then just have a fighting chance of getting off the inevitable charge...nudge nudge wink wink.
However, did you come here looking for AD as in Air Defence?