[Easy]

Human skin (specific weight of 1.09) is considered very resistant to ballistic injury and has required a lot of research over many decades to establish a sound database of ballistic evidence. It must be noted that this study does not deal with penetration of the skin but only with surface injury. Generally, as the bb projectile begins to impact skin, the retarding force of the skin itself causes it to decelerate and lose kinetic energy. This rapid deceleration causes the bb to deform as it expands against the skin surface thus (a) increasing its cross-sectional area towards the impact axis and (b) transferring more of its kinetic energy into the HSST. Softer bbs will deform more readily and will therefore transfer more energy to the HSST and over a greater/deeper area than will harder bbs. An impacting bb causes crushing, laceration, stretching and contusion of the tissue in front and around it. There are many models used to represent the size of the wound, one of the simplest to understand is expressed as follows:

Ed = Cv*V

Where:
Ed - is dissipated energy
Cv – is a constant depending on the properties of the target material, in this case skin
V – wound size or total inflicted area

Therefore the size of the inflicted area is directly proportional to the dissipated energy Ed. The dissipated energy depends upon the time the bb remains in contact with the skin. We know from the laws of physics that (1) the result of any impact between two objects depends on the force and time during which the objects are in contact; (2) the time the two impacting objects remain in contact depends on the material properties of the two objects; (3) the softer the objects the more time they will remain in contact. Soft bbs impacting on HSST will remain on the impact zone for a longer period of time therefore dissipating more energy into the HSST and causing bigger and deeper wounds