The second half of "Arms and the Boy" talks a lot about teeth, claws, and sharp objects (antlers and talons) used for killing, eating, and that sort of nasty thing. But these are all things that human bodies don't have. Which is kind of the point. Humans aren't made for killing. Machines and animals are.
Lines 7-8: The speaker describes a bullet cartridge with "fine zinc teeth." Teeth are a metaphor for the bullets, which are sharp and used for killing. The repetition of the word "sharp" reminds us of the wounds of war, and of how bullets can so easily pierce flesh.
Line 9: The boy's teeth aren't sharp. They seem made for "laughing around an apple." In other words, he's way out of place in a battle zone.
Line 10: There are "no claws" behind the boy's "fingers supple." Supple means soft, and hints at the boy's gentleness and innocence. Claws don't actually "lurk," so this is—yep—yet another example of personification.
Line 11: The speaker says "God will grow no talons" on the boy's heels, which is a metaphor for the fact that he will never become a brutish animal. Talons are used for eating and killing, which means that in these lines, they're being used as symbols of murder and death.
Line 12: God will also not grow "antlers" on the boy's head. Another metaphor? We think so.