Shooting the Killer Bullet will let it explode to create deals of damage to a character. Touching him will hurt him.
In the NES and Famicom versions of Mega Man, they are light colored. They are light blue, apricot and pink. In the remake version of the game, Mega Man: The Wily Wars, they are aqua, dark apricot and red.
In Ruby Spears' Mega Man, Killer Bullets only appeared in the opening scene. They have never appeared in any episode. Killer Bullets and Napalm Man are the only villains that never appeared in any episode.
In Captain N: The Game Master, a Killer Bullet is orange and has black stripes. It destroyed Pit's vehicle. It only appeared in Episode 4.
In the cartoon, Mega Man: Upon a Star, Killer Bullets were seen in the mob along with Pipis, Gyro Man, Crystal Man, Star Man and more. Mega Man, Proto Man and Beat travel to the robot city to attack the mob. After Mega Man knocks out Elec Man and Wave Man, he fights a Killer Bullet, Octopus Battery and Pipi.
If you select a stage in the arcade game, Marvel vs. Capcom, there is a giant Killer Bullet.