Bingo. Therein lies the answer, being trained. Although a little common sense goes a long way.
Anybody who owns a gun, and has not gotten formal training which includes the legal aspects of self defense, is a fool
At the minimum, buy a freaking book. I strongly recommend In the Gravest Extreme: The Role of the Firearm in Personal Protection (Paperback) as the best $12.95 a gun owner could ever spend