Let the boy try along this bayonet-blade How cold steel is, and keen with hunger of blood; (1-2)
The fact that the boy needs to experiment with or feel along ("try") his bayonet-blade makes him seem inexperienced, innocent. He doesn't really understand yet what his weapon feels like, or what they are for. Hey, he'll learn soon enough.
Lend him to stroke these blind, blunt bullet-heads Which long to muzzle in the hearts of lads. (5-6)
It is almost as if this kid has never even held a gun or touched bullets. Yikes. We're thinking he might not make the best soldier at this point. Also, check out the language the speaker is using here. It sounds an awful lot like he wants the boy to "stroke" the bullets, to love them almost as if they were his girlfriend. Could he be talking about another kind of innocence lost as well?
For his teeth seem for laughing round an apple. There lurk no claws behind his fingers supple; (9-10)
The boy's "teeth" are different from the "fine zinc teeth" (7) mentioned earlier. Those are for killing, but his are for "laughing" or eating an apple. The boy is like the cartridge but he is innocent, made for fun times, not for killing.
And God will grow no talons at his heels, Nor antlers through the thickness of his curls. (11-12)
According to the speaker here, humanity will never lose its innocence. It will never magically morph into a killing machine to begin with. The "talons" seem to represent a weapon of some kind. God will never give the boy "talons," which is another way of saying humanity will never become biologically equipped for killing.