In what way is courage, in this case, something like a man with an empty gun?
Atticus tells the children on the last page of Part 1 that he wants them "to see what real courage is instead of getting the idea that courage is a man with a gun in his hand." Thanks.
When Atticus shoots the mad dog, he is in complete control of the situation. The dog has no chance against a marksman with a gun. Mrs. Dubose, however, is like a person fighting her battle with an unloaded gun. She has no chance of living a longer life, so she picks a slightly different battle that she can win: overcoming her addiction to morphine before she dies. Atticus points out that real courage is
"when you know you're licked before you begin but you begin anyway and you see it through no matter what."
In Atticus' view, it took no courage to kill a defenseless dog. However, Mrs. Dubose showed real courage by taking seeing through her battle to the end.
Excellent responses so far. All I would add is that the children, Jem in particular, had resented Atticus for his seeming lack of manliness. Jem bemoaned the fact that Atticus would not play football with him. Their father, an expert marksman ("One-shot Finch"), performs a tough shot under pressure--rabies is almost invariably deadly to humans--and in doing so, inadvertently provides a violent role-model to the children. It is ironic, therefore, that Mrs. Dubose soon provides an entirely different type of role model for Jem and Scout.
In Chapter 11, Atticus is saying that real courage is what Mrs. Dubose has done -- it is not what he himself did in the previous chapter when he shot the mad dog.
I think that he is saying that a man with gun in his hand has power. What such a man does (when he shoots a dog) is not courage because he is kind of in control.
By contrast, what Mrs. Dubose does is an example of courage. All she has to fight with is her willpower. She is fighting against something potentially stronger than she is and she does not have any big weapons.