Most brass shot from fighters was expended through chutes or ports under the wings or fuselage, this model shows the underside of a Hurricane. The black rectangles represent the spent ammunition chutes. Spent ammunition from bombers was generally collected in bags, note the accordion bag on the right of the MG of this He-111's nose gunner.

The fighter's spent ammunition would be next to impossible to retrieve since they fall out of the plane immediately after being shot. As for the bomber's, I'm not entirely sure and speculation helps no one.