What is the effect of a gunshot from 12-18 inches on a surface?

Answer 1

That will vary depending on the nature of the surface, and the type and caliber of the firearm.

Concrete or cement or rock: projectiles at high velocity will cause pits to be chipped out of the concrete. This is known as "spalling", and is easily observed by looking at many of the military buildings around Hickam Air Force Base, Honolulu, Hawaii. Machine gun bullets knocked chips or "spalls" out of the concrete walls of many hangars and of the Base Operations building.

Metal surfaces: depending on the metal and the type of projectile, the metal may be pierced with a ripped appearance, or the projectile may bounce off of the surface. For very hard surfaces with soft lead projectiles, the projectile may be "pancaked"as it deforms and slows down.

Wood: depending on the thickness and hardness of the wood and the caliber of the projectile, the bullet may penetrate the wood completely or partially. The projectile may be caught by the wood. The surface may splinter, ejecting high-velocity wood splinters either back toward the shooter or forward through the wood.

From a distance of 12-18 inches, ricochets back toward the shooter are highly likely.