Back Face Signature – shortened to BFS and also known as Back Face Deformation – is the maximum permissable indent into ballistic backing material after impact. It is measured because it is important that body armour not only stops a bullet penetrating but also protects you from damage caused by the energy from the projectile.
When a bullet hits body armour the energy is so great it will create an indent.That indent needs to be as small as possible otherwise you can sustain serious damage to your internal organs. Therefore, when testing is conducted on body armour it is placed on top of a backing material, usually ballistic clay, and the indent is measured after being fired upon.
The maximum permissable indent is 44mm from a single handgun shot and 33mm from a rifle. A number of shots are fired at the same sample during testing and a maximum average of 25mm is permitted for rifle rounds. The table below is relevant to UK standards and published by CAST (formerly HOSDB).
Back Face Signature Injuries
The energy transferred through armour to your body is so great that serious internal injuries can occur. To minimise those injuries it is important to buy armour that conforms to standards and has been vigourously tested by an independent company. Wear armour that is right for the threat you could face and consider the use of trauma pads to dissipate the energy absorbed.
A great paper, published in 2005 entitled “Modeling Gunshot Bruises in Soft Body Armour with an Adaptive Fuzzy System” (link) states;
Ggetting shot with a .22 caliber bullet when wearing soft body armor resembles getting hit in the chest with a 40 MPH baseball. Getting shot with a .45 caliber bullet resembles getting hit with a 90 MPH baseball.”