The title tells us what we're looking at: a spent bullet casing. The poem never says so directly. Instead it tries on four other images, each paired with a question: a bottle, an organ pipe, a flower bud, a shed insect skin. Every one of them is something that once held or released something else, and every question asks about what's now missing.
The speaker sounds curious, but under that there's a quiet unease. The bottle has been drained of "amnesia," as if the casing has forgotten what it did. The organ pipe's "so high a C" suggests the sharp crack of a shot. The bud and the red petals hint at blood, though the poem leaves that unsaid. The last question is the darkest: a creature wouldn't leave such a "narrow corpse" willingly. The mood is hushed, clipped, and haunted, with the violence felt only in what has been emptied out.