回复：如何算合法用枪自卫？

that really depends on the state you lived. In general, you can ONLY use deadly force to protect your friends or your family members when their lives are threatened. The word "Threat" is a word which can be debated. In other words, your lawyers mush provide the evidences where your live is threatened. One question you will be asked in the court is - why you did not run away rather than using deadly force. You need to prepare the answer to prove that "using deadly force" is your only choice. Therefore, two conditions you need to concern.

1. Live is threatened.
2. No other option is available at that time.

I can't give you the right answers. Instead, before you pull the trigger, you need to make sure you have the answers for above questions.