Symbolism, Imagery, Allegory
When Matt's dad gives him the rifle, Matt knows the whole hold-down-the-fort-while-I'm-gone plan is for real. Eek. He also knows that his dad doesn't feel as hunky-dory about it as he pretends to. When he gives his son the pricey gun, he warns Matt not to "go banging away at everything that moves. Wait till you're dead sure. There's plenty of powder if you don't waste it" (1.19). These words remind Matt that he's still an immature kid, and his father feels "uneasy about leaving [him] […] alone" (1.20), so Matt vows to treat that rifle with respect. It is one way Matt can prove to his pa (and himself) that he's a man inside.
The rifle doesn't last long under Matt's watch, though, and when Ben steals it from him, it's a painful reminder to both Matt and the reader that he's pretty much just a kind all alone in the woods. Though Matt will be able to survive on seafood, Ben has totally swiped his pride, and the poor kid is haunted by wondering what his father would say (3.59) if he were there. Of course, Matt's dad isn't there… which is why he has the rifle in the first place.