His mother Elizabeth later uploaded a picture of her son to Facebook with the message: "There are no words".
No charges are believed to have been filed against Mr Brumby in what has been described as a tragic accident.
"After firing a round, the spent shell casing struck the wall causing it to deflect and fall into the back of (the elder) Mr. Brumby's shirt.
"Brumby then used his right hand, which was holding the handgun, in an attempt to remove the casing. While doing so, he inadvertently pointed the firearm directly behind him and accidentally fired."
A few weeks before the accident, the gun store commented on its gun safety programmes in a Facebook post saying, "Properly trained kids don't have accidents."
But after Stephen died, the owners didn't want to comment on camera, only offering condolences and prayers to the family of the teen killed, reports the ABC.
The tragedy comes weeks after the mass shooting in Orlando, Florida, where a gunman killed 49 people at a nightclub. Florida is among the easiest states in America to buy a gun.