Touchy topic here. I'll input my two cents. I own an Akita, so I'm familiar with big dogs and handling them when they get rowdy. It's pretty rare that I have a firearm handy, so I'd plan for defense in other ways. Much as most everyone else on this forum, I almost always have a knife with me if I'm not in the house. As soon as Fido starts his charge, that knife is going to be in my hand.
One thing I've noticed from playing with dogs in the past, especially large breeds like a German Shepard or the Akita, is that while they're running at you really fast, if you hold your ground and dodge to the side at quite literally the last second, they take a second or two to get turned around. There's your opportunity. Do what's neccesary.
I'm a pretty big fan of non-lethal dissuasion, but there's a bit of a catch-22 there. On the one hand, I'd get really, really, REALLY mad if anyone messed with my dog. On the other hand, if my best friends boxer ever charged me....well, lets just say I hope he understands that I put my well-being before that of his pets. However, if for some reason you do not wish to use lethal force, I've dicovered it's not particularly difficult to put 85 lbs of Akita on the ground and keep her there with one arm. As she charges, you try to get positioned so the dog is on your left, assuming you're right handed, and just as they reach you, side step, drop to a knee and throw your stronger right arm around their throat. Dog comes up and over your knee, hits the ground with you on top of them, and stays put.
All this said, a ticked off animal that you've hurt is bad news, no matter how you slice it. Sorry about the long post, but I really like dogs.
-Kyle <img src="/images/graemlins/laugh.gif" alt="" />
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"The object, gentlemen, is not to cheat death: the object is not to let him play."
-Sgt. Poteen