This is how ballistics will find match for gun that killed Brendon Mackey

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CHESTERFIELD, Va. (WTVR) -- Officers say former firefighter John Gardner's firearm is the eighth one they've tested since Brendon Mackey was killed.

Chesterfield police said the bullet that struck and killed Brendon Mackey was fired from a .40-caliber gun.

As first reported by CBS 6, police were called to the former Chesterfield firefighter's home about three in the afternoon, a week ago, after neighbors reported hearing shots fired.

According to an affidavit, a woman who was inside the house told police it was Gardner who fired the gun. Officers then entered the home to check on his welfare, and found a gun lying next to him, with a bullet ricochet mark on the tile floor and a bullet hole in the wall, according to the affidavit.

Chesterfield police forensics officers spent hours removing bullet fragments from the wall, along with other evidence.

The search warrant shows the following items taken from Gardner’s home:

  • Seven bullet fragments
  • Bullet casing
  • A Glock 23 handgun
  • Glock magazine
  • Nine .40 caliber rounds
  • One loose .40-caliber round

Bullets fired from guns have distinct markings, as unique as fingerprints. Once the state lab has one bullet, all they need is a gun, a test fire, and time to compare the two projectiles.

Almost every bullet fired from a gun, can be traced back to that gun using a microscope.

“When a bullet is fired from a firearm, when it travels through the barrel, the barrel leaves microscopic markings on the bullet that are unique to that specific firearm,” Jessica Wade, forensics firearms examiner, said.

The key is once a bullet is found at a crime scene investigators then have to locate, the same caliber gun. Then both the bullet and the gun are sent here to the state lab for testing.

“We'll examine the bullet for those microscopic markings, and then if there is a specific firearm, that you think may have been used in the crime, we will fire, unfired ammunition from the firearm, collect the bullet and we'll compare the bullets from the firearm, to that one found at the crime scene,” Wade said.

“It is probably the second most popular cartridge behind 9mm in defensive handguns,” Sean Swineford, firearms instructor said, about the .40 caliber weapon.

"When it's closed and the magazines inserted, it will hold one in the chamber, 13 in the magazine-- in the Glock 23. Fires between 168 and 180 grain projectiles about 1,100 feet per second,” Swineford.

It can take anywhere from several days to several weeks for these tests to be complete.

STICK WITH CBS 6. We broke this story, and will continue to provide updates as soon as they are available.