In Dead End in Norvelt, guns have extraordinary levels of responsibility attached to them. They can be used to kill people (as in war), defend your home, or even to feed a family through hunting. They're not positive or negative in themselves, but, as with all the other symbols, good or bad depending on how they're used.
When Jack fires the gun accidentally at the beginning of the book, his dad sees it as a sign of immaturity. (Gee, pot calling the kettle black, much?) Jack hasn't listened to all of those gun safety lessons his dad has tried to drill into his head, and so he's grounded—exactly like a kid who's done something stupid.
That's why, when Dad gives Jack the bullet from the deer at the end of the book, the gesture implies that Jack has crossed some kind of momentous threshold (28.6). He's become a man.
Which brings us to another point: we mostly see men using these tools, so the book really associates guns and bullets with dudes and their dudely duties. Even on the two occasions that women use guns (Mrs. Dubicki chases Mr. Spizz from her property, and Jack's mom calls for the Japanese sniper rifle when she confronts the poacher), they're using their husbands' guns and are only temporarily filling in a job that their husbands would normally do. That makes guns a powerful symbol of manhood and responsibility—like cars, but even more deadly.