Here is what the production notes say:
One of the most important design elements of the film was that of "The Mexican" itself. "The design of the pistol was critical because it becomes almost a character in the movie," Gore Verbinski states. "I wanted it to be unique, with a chamber in the shape of a heart, which presented some design issues. I envisioned it something like a jewel box, so you almost had the feeling that you could wind it up and it would play a little tune ... yet it can kill you."
The pistol was built by Neotek, Inc. based on an original design from the Museum of History in Mexico, as well as sketches and design concepts from Verbinski. In addition to the heart-shaped chamber, the director created the snake coming around eating the apple and, on the hammer, the man coming up the side holding the woman, so it was obvious that this was a lover's gun. Verbinski reflects, "My concept for the pistol is simply that love is worth fighting for."