[jogger08152]

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When she initiates violence against you. In this instance, it would be the moment she squeezes the trigger (while pointing the gun at you, not her own head).

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But what if there are no bullets in the gun, or only blanks?

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She has tried to attack you and failed. (Veracity test by extension: if everybody walked around with empty guns, pointing them at random and squeezing the trigger, (however you might feel about this behavior) should they be arrested for "attacking" others? I don't think so.)

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Would I be justified in tackling her and breaking here arm to halt further attack, even though she couldn't possibly have harmed me?

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Not according to ACism, as I understand it, since she has not actually attacked you. You might well want to do so anyway, however, and a more reasonable system would certainly permit you to do so, since (assuming she did not know that the gun would not fire) she is plainly a very real threat to you, and more reasonable systems allow violence in response to a mere threat, not merely in response to violence itself.

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Does it matter if I perceive to be attacked, or if I actually am attacked?

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The following answer assumes you are asking what is/is not allowable according to the ACist claim that one must never initiate a force transaction: If you perceive an attack that didn't happen and initiated a force transaction in response, you would be morally wrong, mitigated by the fact that at the time you initiated your force transaction, you did not believe that that was what you were doing. (Ala the policeman who shoots a kid holding a water gun, because he erroniously thinks that it is a real gun.) If you actually are attacked, you have done no wrong by closing the force transaction.