When Brian recovers the survival pack from the plane toward the end of the book, one of the things he finds inside it is a ready-to-assemble rifle. You know, the kind that—oh wait, we have no idea what one of those looks like.
But instead of being glad that he now has a weapon to help defend himself and find food, Brian reacts to the gun with discomfort and uneasiness:
The rifle changed him, the minute he picked it up. (19.8)
Without the rifle he had to fit in, to be part of it all, to understand it and use it—the woods, all of it. With the rifle, suddenly, he didn't have to know; did not have to be afraid or understand. (19.7)
The rifle, unlike the hatchet, is a tool that's easy to use and (gulp) easy to kill with. As Brian realizes, a man with a gun won't really have to get to know the animal he's hunting in the same intense, time-consuming way that Brian has to in order to catch his prey—he can more or less just point and shoot.
What does this mean? Although it might help Brian survive, the rifle also threatens to distance him from the life he's made in the woods, and from the real, in-the-moment sense of belonging represented by Brian's encounter with the wolf. The rifle separates Brian from the feeling he has when he sees the wolf ("[Brian] knew the wolf for what it was—another part of the woods, another part of all of it" [13.8]). It takes him out of the natural world, and makes him a stranger, no longer part of what's around him.