If you have to use lethal force make sure to use good judgment.
I'm assuming, by the way you wrote the post, jhco, that you're not the kind of guy who would shoot a dog that's rolling over at your feet, on its back with a big grin and its tail wagging. There are people on this board who think that's "aggressive behavior".
That said, a dog that is bred to be an attack dog not only will not back down when you pull out a gun, it will also not back down when it is shot.
Bulldogs, mastiffs, etc. don't respond to pain like we do. There are stories of dogs with their guts ripped out, still hanging onto bulls. It's not appropriate to use a choke chain or slip lead for training on these dogs, because if they get it in their head that there's something they want on the other side of the street, they'll just hang themselves, pulling with their own strength against the choke lead.
And most of these dogs (true Pit Bulls included) were not even bred to be aggressive towards humans. They're just bred to have a one-track mind and not let anything stop them. A dog that really is bred/trained to attack humans is a truly scary animal if it's out of control.
So, there's something really important. If you are dealing with a truly, viciously-aggressive dog, a gun shot that just causes pain will likely not stop the attack. If it does, then the dog probably wasn't much of a threat to begin with. That's why OC doesn't work in this situation.
If you shoot, you have to shoot to stop. Not to discourage. Think grizzly bear tactics, but a smaller animal.
If you haven't seen this video, watch it. These appear to be Belgian Malinois.
http://video.google.com/videoplay?docid=-3383695521261916155
The Malinois is not even a big dog, and presents a tiny target.
http://en.wikipedia.org/wiki/Belgian_Shepherd_Dog_(Malinois)
From this video, I'd say this. If the dog has this athletic ability and REALLY wants to attack you, you'd better be a damn good shot, and start shooting when it's 20 yards or more from you. And even then, you might as well kiss it goodbye.
One more thing. My wife was at the dog park and raised a chuck-it to throw a tennis ball for one of our dogs. A retired police dog, a huge German Shepherd, came over and grabbed her raised arm and wouldn't let go until its owner told it "release". He apologized, and she was unhurt, if a bit shocked. A trained police dog that's under command is both less, and more, of a threat than many other animals. It depends on who's giving the commands.
Also, someone who is having his dog attack you is committing assault with a deadly weapon, at least in most jurisdictions.
Just some thoughts...
