I also have to say it was abusive of him to break into the bedroom where you had gone to be alone , however you still were both out of control and its not good with children around .
Domestic abuse works both ways. Just because you are "the weaker sex" does not mean you can slap someone around. It's your tough luck that he is stronger than you. The episode of the nerf gun you admit was an accident. Case closed.
I totally agree with Margy. You both seem to have anger control issues - since it wasn't really your "decision" to hit him, maybe it wasn't really his decision to hit you either - just that neither of you has impulse control.
It sounds to me as if you both need some help...are the children around when all this is going on ?Is this the first time he has hit you ?