Looking at a Bond Derringer, for hidden carry. What caliber do you recommend?

I'm looking for a hidden carry Derringer. I have space in my work boots to slide a Derringer that will not be detected. It's not illegal for me to carry at work, but it's better that I don't print so my daily carry XDS .40 S&W gets left in my car. With all the crazy work place shooting that happen, I'd rather go down fighting back so I can be admitted to Valhalla, than cowering under a desk.So what is the best caliber to get in a Derringer? I don't want a 9mm, I have no respect for it after watching Police Activity on YouTube and seeing how ineffective it can be if you miss a vital organ. via /r/CCW http://bit.ly/2IfA4MT