Minimum
To me it is what you can handle and shoot well. I agree with the guys that say a .17, personally my deep undercover is a 2 shot High Standard Derr. in a wallet holster in .22 mag, carry is a Glock 23 (.40) with Crimson Trace's. My wife carries a .380 Seecamp, my daugther a Glock 27 (.40). We all have choices of just about anything on the market but those are the weapons that feel the best to the person. Nothing works unless you hit your target. The best allround (cal., reliabilty, cost, ease of operation) SD gun in my opinion is the .38 revolver. Home defense is a 18" cyl. bore pump shotgun (Mossberg, Remington etc.) with buckshot. Thanks for letting my useless two cents worth into the thread.