In case you didn't notice, the gun in this book plays a big role. It causes a bunch of fights between Sig's parents, and is a major feature in his journey to figure out whether he wants to be like his mom, his dad, or both. This isn't just any gun though—it's an 1873 Colt Single Action Army, a.k.a. a revolver.
We're told, "The Colt is the finest machine I have ever seen in my life. It does one thing, and it does it superbly well" (18.38). So it makes sense that the book would be named after the gun as well—heck, we even get a whole chapter (18, if you missed it) dedicate to exactly how the gun works. Sig's dad wants to be sure his son knows exactly what happens when he fires the gun so he can use it responsibly.
In a lot of ways, the revolver is about Sig learning more about the world, and figuring out his place in it. We've got plenty else to say about this, so hop on over to the "Symbols" section to unpack this book's title in all it's glory.