Several distinct shots are heard during the prologue. Rey Curtis picks up a shell casing from the dock and remarks it looks like a "9mm auto". Later he asks Carolyn Trang if she owns a "Glock semi-automatic pistol". An automatic weapon produces multiple shots with one trigger pull; this is obviously not the case as separate shots are heard. A modification can be made to produce an automatic effect, but almost all handguns sold in the US are semi-automatic.
It is never explained why Todd Wexman pretended to be Larry Rice.