Asked my nephew (an undersheriff who has been through the FBI academy) what his thoughts were on this. He says, somewhat tongue in cheek, that the FBI's idea is that unless the caliber starts with .4, it is ineffective.
With modern ballistic technology, and the availability of decent ammunition specifically designed for self defense, just about any caliber can be used.
Unless it was all I had available, I certainly wouldn't want to depend on anything less than a 9mm. I have both 9mm and .45acp at present and carry either a G19 or 1911 as circumstances dictate.
This is a subject that will generate many differing opinions. Use what you have, and with what you have, practice, practice, practice and more practice. That way any disparity in caliber between you and the bg's becomes a minimal factor in any armed confrontation.