First thought. It wouldn't have happened if Barney had kept the bullets in his shirt pocket just like Andy told him.
Ping for later.
Reminds me of the scene in the Three Stooges movie Disorder In The Court, when Curly is holding the gun and says, "See, I told you it wasn't loaded" just before the gun goes off and grazes some bald guy's head.
Bullet!He only had ONE Cartridge!