Which among the following is the causative factor for forming 'Gutter wound'?

Bullets may graze or rub the top and sides of the cranium without entering it. In such cases, entry and exit wounds are found on the scalp about two to three cm. apa, while the skull between the two wounds shows an oval or elongated gutter-like depression, known as 'gutter wound'. Ref: The Essentials of Forensic Medicine and Toxicology by KS Narayan Reddy, 27th edition, Page 201.