Leveraging body mechanics and available support to provide a stable firing platform for the weapon. Firing positions vary greatly according to situation and available cover, but the shooter must always strive to minimize unwanted movement by achieving the most stable and relaxed firing position they can and maintain consistency in shoulder and cheek welds in order to ensure proper sight picture and sight to target alignment and recoil management.
In almost all firing positions, the head should maintain an upright alignment. A proper cheek weld is established by placing the stock high enough on the shoulder to allow for an upright head position. The stock of the rifle should be brought up to the head, rather than bringing the head down to the rifle. Depending on individual facial structure, the cheek bone is placed directly on the stock, which often results in the skin of the cheek being bunched up and forming a small fold over the rifle stock.
For information about stability with a handgun:
The Handgun Firing Position - Natsoc