Creeping Incrementalism:
I was going with the cow skull/rifle logo a while back, to sort of keep the cover design similar to EFAD's Gadsden snake/rifle design, but in the end I decided not to use it. From my point of view, the main purpose of the cover is to help to sell the book, and to do that it must (1)be very clearly understandable even at thumbnail size, and (2) it must convey some of the meaning or content of the book at a glance. The EFAD Gadsden snake/rifle logo clearly tells a reader at one glance that the book is about guns and the RKBA.
However, in the end I thought that the cow skull/rifle logo didn't do this effectively. It could have been, for example, another Tony Hillerman mystery set in New Mexico. It conveyed "Southwestern" but not the entire idea of the reconquista of "Aztlan" and so on.
I think that this cover design is not as "artistic" as the cow skull, but it better communicates what the book is about. Folks surfing the internet may spend five seconds looking at a 2" high thumbnail image of a cover, and to me, the cover needs to clearly communicate what the book is about in that five seconds. That's why I went with the "torn USA" design.
Matt