Cars are a big no-no in the hunting world. After all, you can't really hunt on an even playing field if you're shooting from a car. The car gives humans too great an advantage. Isn't it enough that they already have guns?
In "The Short Happy Life," several highly-charged moments take place in the car. Margot kisses Wilson when he kills the lion, after Macomber has proven himself a coward. Margot gives grief to the men for shooting buffalo from the car and yet she kills Macomber from the car herself.
The "doorless, box-bodied motor car" (2.63) is a place where the cowards remain – or where they retreat to when things turn dicey. Macomber is reluctant to shoot from outside the car because the mobility it provides is a comfort to him. It is an escape route from the threats of the African plains. In a moment of chivalry, he later tells Margot to stay in the car while he goes to look at the lion.
But if the car is associated with fear, what does Hemingway suggest by having Margot shoot her husband from the car rather than outside? She has just been harping on the men for shooting the buffalo from the car, and now she is doing the exact same thing. Is she afraid of the buffalo? Afraid to shoot her husband point blank (but just brave enough to do it from many meters away)?