A three-year-old boy found a loaded handgun in his garden before his 11-year-old brother fired it at a car.
The boys had thought the weapon - found in Oakfield Close, Smethwick, on Saturday morning - was a toy.
Nimco Guled, the boy's mother, 38, said the people who dumped the gun in her flat's garden were "evil and selfish".
It comes as Home Secretary Jacqui Smith visited the new anti-gun crime agency, the National Ballistics Intelligence Service (NBIS), in Birmingham.
Miss Guled, who is originally from Somalia, said the gun was found by her three-year-old son Mahde Ali.
'No brain'
He then gave it to her 11-year-old son Warsma who fired it at a car.
Referring to the people who dumped the weapon, Miss Guled said: "Luckily it was the car got hit. They don't have a brain, they don't have children, they are just evil and selfish."
"They knew this area was a family area, they 100% knew.
"They don't even think there's children living in the house and tomorrow they're going to be playing with it."
The area was cordoned off and the weapon is being examined by experts.
Earlier, Ms Smith visited the £8m NBIS, which includes a national database for recovered guns and ammunition.
She said: "I am determined to tackle gun crime and get guns off our streets."
Birmingham, Manchester and London are identified by the police as experiencing disproportionate gun and gang crime.
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