"The Death of the Ball Turret Gunner" uses intense metaphors of wombs, dreams, and awakenings to evoke the speaker's innocence—and his terrible death.
Right from the start, the poem metaphorically connects the speaker's experience as a ball turret gunner to the experience of being inside a womb. This becomes particularly clear when the speaker says that he "hunche[s]" in the "belly" of "the State," creating an image of himself curled inside a living thing. In this image, the ball turret (which here represents the government) is alive. What's more, it has taken over responsibility for him. In other words, he goes straight from his mother's womb to the womb of the ball turret. When he metaphorically "[falls] into the State," it seems as if he's just stumbling into this war, defenseless and vulnerable as a baby.
And like a baby in the womb, he seems to be unconscious—until he wakes to find himself "Six miles from earth" in a warplane. Up here, he's "loosed" from the "dream of life"—that is, the illusions of life down on earth. Because the poem is about what it was like for naïve young soldiers to go off to World War II, perhaps this "dream of life" is a metaphor for the way the US government drew people into the military with unrealistic visions of valor, romanticizing war and turning it into a fantasy (a "dream") of courage and heroism. That dream ends when the enemy starts shooting at the speaker; in contrast with the metaphorical "dream," those enemies are part of a new, terrible "nightmare."
The poem's final line ("When I died [...] a hose") builds on the metaphor of the speaker as an unborn infant, this time subtly suggesting that he died in the womb and that his new mother (the ball turret) has a stillbirth or abortion. This only emphasizes the fact that he never truly got to act for himself. Like an innocent baby, he has no way of defending himself, and none of his protectors (neither his mother nor "the State") end up keeping him safe from the horrors of war.