david mattingly on what prosecutors may be trying to prove with zimmerman's use of it. >> reporter: listen, if you're following the george zimmerman case, you probably heard this sound before. [ gunfire ] that's the sound of a shot from a keltic pf-9 handgun, just like the one caught on a 911 call the moment george zimmerman killed trayvon martin. >> there's gunshots. >> you just heard gunshots? >> yes. >> reporter: zimmerman carried the gun legally. but prosecutors seem to suggest he was doing more than that. >> it wasn't necessary for the defendant to rack it to load a round. it was ready to go. >> reporter: zimmerman was carrying the gun with the bullet already loaded in the chamber and ready to fire. was this a sign he was planning to do harm when he encountered trayvon martin? gun safety inspector larry holt told me zimmerman was doing exactly what he was supposed to do. was he carrying it properly?