Public domain images. Click on images to enlarge.
In the two pictures above, we see two men in different prone positions. In the first one, the man is positioned in line with the axis of his rifle barrel and in the other one, we see the man lying almost perpendicular to the line of his rifle barrel. The feet may also be placed close together or splayed apart.
Regardless of which position is used, this is the best position for maximum steadiness. However, if the person is walking with a rifle, then getting into this position takes longer than the standing position or kneeling position. It is one of the standard positions that is taught by all militaries of the world.
It is not advisable to use this position when handling a large-bore rifle, or one that has huge recoil, because the user risks snapping their collar-bone, due to the fact that the butt of the rifle sits higher on the shoulder than when in the standing or kneeling position. It is also not useful, if the ground in front of the shooter slopes steeply up or down, or if ground cover (bushes, shrubs, grass etc.) blocks the target. However, it is the best position for maximum accuracy and also allows the user to use any bit of available cover.