Aristotle said that one of the main uses of metaphor is to discuss thing beyond the experience of the audience. If your audience has no direct experience of the subject of the poem, then it is difficult to use direct language, but you must rely on comparison. What this poem is trying to convey is the experience of being curled up around a machine gun in a “ball turret”, a glass sphere attacks to a plane. On the one hand, the position is almost fetal, and seems comforting; on the other hand, imagine being in a transparent sphere 6 miles above the ground. Then try to imagine what happens when an enemy attacks the plane. The description of the remains of the body needing to be cleaned out of the shattered turret with a hose is graphic and unambiguous.