The NAA guardian .380 is a great little gun. Search on the forum, I wrote a small review on it. It kicks like a mule - much more power than expected. The gun is made very well and will serve you well. I would recommend carrying something in .40 caliber or higher as a primary gun, but the .380 is a great BUG (back up gun). Good luck, if you have anymore questions, feel free to reply or private message me.
.38 is the smallest I would recommend for a primary, but it would stop most threats. I would look for a revolver that accepts .357 in addition to .38 that way you could still go out plinking for cheap, but carry a round with a little more punch incase you ever had to use it.