A contact shot is a gunshot made with the muzzle in contact with the victim.
When the muzzle of the gun is in contact with the victim, everything that comes out of the barrel impacts. Things that dissipate over short distances like hot explosive gasses, gun powder soot, etc. all strike the victim. As a result, a contact shot is relatively easily determined forensically. Burning or singeing around the wound, local discoloration from burned powder, tearing of cloth and skin from the gasses, abrasions from the muzzle, etc. all indicate a contact shot.
Imagine the size of the explosion required to accelerate an half ounce hunk of metal to a speed of nearly 1,000 feet per second over 5 inches of metal tube. Right behind that hunk of metal are the hot gasses and explosive by-products that propelled it. In a contact shot against a strong bone, the stuff behind the bullet can tear away skin with enough force to create flaps or make an impression of the muzzle. I encourage you to not look for photographs.
BrevityQuest12, 177 words