If someone was robbing you, I doubt you could be like, "Hold on one second kind sir, I must take a swig from my flask!" before he either robbed you, stole your shiny and expensive looking flask, or just shot you.
And a derringer in short, sucks. You can barely fit them in your hand, and they put out less than powerful rounds. I wouldn't suggest carrying one unless it's a last defense backup gun.
I'm with Matt on this, something like a subcompact Glock or even a standard handgun usually tends to be the best carry gun, and as for being searched by the cops, if you're legally carrying, all you can do is bite the bullet and put up with their shit.
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