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)
Frankensence: Yo man what are you saying tonight
Mur: I got a beretta
Frankensence: sesh at my place?
Chris: Got yo gat?
Chris: word, best reach.