Stemming from Dr. Dre lyrics, to "have a beretta" is to be open for chilling, down to hang out, not busy, or free.

from 2001s "The Watcher", Dr. Dre's lyrics go as such:

"as long as I got a beretta,
yo, I'm down for whatever"

thus, if you ask "what's up" and someone replies, "I got a beretta," they are essentially saying, "I'm down for whatever.

It's a good way to sound cool when caught with absolutely nothing to do.

The slang further develops when the words are supplemented with synonyms. (see example 2)