Symbolism, Imagery, Allegory
Since Revolver is the name of the book, you probably guessed a gun would play a pretty important part, but the book isn't really about an 1873 Colt Single Action Army (or, you know, revolver) specifically. Before we go any further, though, let's check out one of Einar's little lessons for Sig about the weapon:
"Perfection," Einar said. He paused, and Anna marveled at the look in his eyes, because it was one of love. "Do you see what I mean now? It's perfect, and if perfection is beauty, then this is the most beautiful thing in the world. A piece of man's incredible ingenuity, a machine, perfectly designed around the hand of man." (18.36)
Woah… wait a second. This reads less like a lesson and more like a love letter, doesn't it? And while we certainly learn plenty about guns, bullets, and firing chambers in this book, it's Einar's adoration for these weapons that matters most. The revolver starts a discussion about guns and violence in the book, and serves as a measure for Sig's maturity. While in general guns represent growing up for Sig, his progress is best marked by the shift in him between excitement and all-out enthusiasm to a warier understanding of firearms.
He may be surprised how easy a gun is to use the first time he shoots one, but by the time Wolff is threatening him and his family, he's seen the destruction guns can bring and thinks twice about firing off at Wolff. He's grown considerably, and how he views guns represents this process.
Revolver refuses a neat and tidy assessment of guns. As Sig thinks about what actually happens when people use guns to solve problems, we see a healthy mistrust for firearms develop—but we are also repeatedly shown that guns are an important, and impressive, invention. That's why Einar is so captivated with how they work, and also why Sig immediately sets about trying to get his gun when Wolff shows up.
Eventually, Sig lands somewhere between his parents when it comes to guns. He doesn't idolize them as perfect pieces of machinery like his dad does, nor does he see them as just needlessly harmful and dangerous weapons like his mom. So the revolver represents critical consumption of his parents' ideas—a key part of the maturation process—and also invites readers to meditate on the benefits and risks of guns in our lives.