A middle aged lady was found in a robbed room lying in a pool of blood on forensic examination there was an entry wound of size around 2 x 2 cm on the left temporal region with tattooing and blackening around the wound. There was also an exit wound with beveling at the right temporal region. On fuher examination two bullet fragments were found inside the brain parenchyma. Which of the following could be used to determine the distance from which the weapon was fired?