Lend him to stroke these blind, blunt bullet-heads Which long to muzzle in the hearts of lads.
The speaker continues to give commands, but we're still wondering just who in the world he's talking to.
He tells somebody to help ("lend") the boy "stroke" the bullets, which apparently really wants to burrow ("muzzle") into the "hearts of lads." Man, that bullet's got some anger issues.
On an obvious level, these bullets are "blind" because they aren't real and therefore can't see. But Owen's also subtly pointing out that bullets go wherever people shoot them. They're not choosy about what they hit.
There's a contrast here with the first stanza: a bullet is "blunt," or rounded at the top, not sharp like the bayonet-blade. But it can do just as much damage, if you ask Shmoop.
Despite the contrast, though, the idea here's the same. The speaker wants this boy to understand that these weapons are bad news—they want to hurt people, shed blood, and do some serious damage.
Or give him cartridges of fine zinc teeth, Sharp with the sharpness of grief and death.
Moving right along to yet another weapon. Here the speaker suggests that somebody—anybody?—give the boy "cartridges" of "zinc teeth" which are sharp with the "sharpness of grief and death."
The cartridges described here are for holding bullets. The "fine zinc teeth" refers to the bullets they contain.
Of course the bullets aren't literally sharp (they're "blunt"), but they are sharp or can "cut" with the grief and death they cause. Grief and death, in other words, are the source of their power. They're lovely things, really, bullets.