to create more effective manstoppers. Therefore, such effects are not as reliable as physiological effects at stopping people. Animals will not faint or surrender if injured, though they may become frightened by the loud noise and pain of being shot, so psychological mechanisms are generally less effective against non-humans. Penetration According to Dr. Martin Fackler and the International Wound Ballistics Association (IWBA), between of penetration in calibrated tissue simulant is optimal performance for a bullet which is meant to be used defensively, against a human adversary. They also believe that penetration is one of the most important factors when choosing a bullet (and that the number one factor is shot placement). If the bullet penetrates less than their guidelines, it is inadequate, and if it penetrates more, it is still satisfactory though not optimal. The FBI's penetration requirement is very similar at . A penetration depth of may seem excessive, but a bullet sheds velocity—and crushes a narrower hole—as it penetrates deeper, so the bullet might be crushing a very small amount of tissue (simulating an "ice pick" injury) during its last two or three inches of travel, giving only between of effective wide-area penetration. Also, skin is elastic and tough enough to cause a bullet to be retained in the body, even if the bullet had a relatively high velocity when it hit the skin. About velocity is required for an expanded hollow point bullet to puncture skin 50% of the time. The IWBA's and FBI's penetration guidelines are to ensure that the bullet can reach a vital structure from most angles, while retaining enough velocity to generate a large diameter hole through tissue. An extreme example where penetration would be important is if the bullet first had to enter and then exit an outstretched arm before impacting the torso. A bullet with low penetration might embed itself in the arm whereas a higher penetrating bullet would penetrate the arm then enter the thorax where it would have a chance of hitting a vital organ. Overpenetration Excessive penetration or overpenetration occurs when a bullet passes through its intended target and out of the other side, with enough residual kinetic energy to continue flying as a stray projectile and risk causing unintended collateral damage to objects or persons beyond. This happens because the bullet has not released all its energy within the target, according to the energy transfer hypothesis. Other