I'm a FedEx Ground package handler, however, the following is not an official opinion of FedEx Ground.
The most likely reason is the person loading the truck failed to properly inspect the ZIP code on the package as they were loading it into the truck. Misloads like this occur from time-to-time, especially with new employees, or when an employee rushes and gets careless, or when they are just not paying attention. The company and its' employees continually strive to provide dependable, timely service with no problems or delays for the customer. However, things happen despite mountains of preventive measures.
On behalf of FedEx and all of my fellow employees, I apologize for any delays getting your pistol to its' final destination. I feel wholly confident that your pistol was not tampered with at any point during its' journey. For one thing, I'm certain the people pushing it along were too busy to even notice what it was. Also, there are too many other eyes, both human and video, in all areas of a FedEx hub for someone to open a package, remove a pistol, and tamper with it without drawing a LOT of attention.
Another possibility is that the original truck was only filled to a small percentage, so they may have put up a load net and filled the back of the truck with packages for another destination. The truck may have gone to the destination for the stuff now in the rear of the truck, where it would have been off-loaded and combined with packages from that hub going to the hub your package was bound for.
Frank
After rereading the route your package took, there is another possibility. Since it went to to LA, then to Phoenix, and back to LA, it's possible the person loading it into the truck misread the shipping sticker in Phoenix and thought it was going from Phoenix to CA, instead of from CA to AZ. Unfortunately, that happens once in awhile when the flow is going pretty fast and furious. Again, I apologize on behalf of FedEx.