I had an incident with a dog recently, Its a long story but maybe it can give you a bit of insight.
I weigh 135 lbs
Last summer, out of an act of kindness, I let a friend stay in my home (who happened to be addicted to heroin but not through her own doing, doctors). The one stipulation for staying in my house while I was in Puerto Rico was that her friend (if he got out of jail on his traffic/gun/drug/whatever/loser charge) could absolutely not come to my house and especially not with his PitBull, for which I had almost been arrested previously.
Unfortunately my trip to PR was cut short by the suicide of a friend. I returned home to find my house trashed by an unconcious girl and inhabited by an insane, gun weilding junky who thought it was okay to be in my house while I was out of town.
When I came to my house, I was unarmed and tired from the airplane ride from PR to Denver during a hurricane. I walked into my house to be attacked by a PitBull who I never expected to be there.
the moment I entered my house the dog and owner became agitated. The owner was obviously on drugs. I asked all three parties, including the dog to leave, at which point I entered my room and drew my weapon.
I asked them to leave again. The dog owner became more agitated, though he did not know i was armed. I re entered the living room where these people seemed to have accumulated and Jason, the dogs owner, told the dog, Prevo to attack me! I drew my weapon and shot the dog twice (Im not terribly fond of that memory) and in turn the owner called the police on myself.
The denver police get there FAST!
They arrested Jason, Let the girl go (she got kicked out) and sent me back to my living room to clean up Prevo.
The point of my story is that IF you really have to defend yourself, from mortal danger, against a dog, (it really has to be life threatening) bring something bigger than a 9mm if the dog is bigger than or appx the same size as a PB.
Just some thoughts,
-FL