"We thought they were fireworks initially, so we were huffing up the stairs putting the fire out and officer Mendez comes up to us and says 'that popping sound you hear those are bullets going off.' So we had to back off," Ty Wilson from the Union City Fire Dept. said.
"Thousands of rounds of ammunition were stored inside the room of origin along with a number of weapons," Assistant Fire Chief Andy Smith said.
Neighbors say the bullets went off for more than 15 minutes. Residents of the homes bordering the fire were evacuated by police until the area was safe enough to approach.
Next door neighbor Jermaine Miller showed ABC7 the exploded shells that littered his backyard.
"It was kind of nerve wracking you know," he said.
The ammunition that went off ranged from 22 and 32 caliber to shotgun shells. Police removed two assault rifles, two shotguns and other rifles and a pistol from the home.
Laura Luke, who lives in the home with her son, says the ammunition and weapons belonged to her father and they were passed along to her brother who kept them in his room.
She says the fire started with an electric blanket. Her brother declined an interview.
"It's ammunition from like 20 years ago and that was just exploding all over the place," she said.
Frightened neighbors from as far as a block away picked up spent ammunition from their front and backyards. A car a block away on San Carlos Way was hit by a round that cracked its windshield and fortunately there were no injuries.