A Michigan grandmother has been found guilty of murdering her 17-year-old grandson after she shot him six times.

The mother of the teenager branded 75-year-old Sandra Layne a "monster" and said she deserved to be jailed.

The grandmother fired 10 shots, striking Jonathan Hoffman six times. She claimed self-defence because an argument had turned violent, the court heard.

The jury found her guilty of second-degree murder and using a gun and faces 14 years in prison.

Layne said she purchased the gun because she was scared of the victim and his friends after they moved in with her.

"It's a final vindication for my son, to restore his good name and reputation, because over the course of the last nine months, it's been tarnished in a very cruel manner," Michael Hoffman said.

He added that his son was dressed only in socks and athletic shorts when he was shot.

The victim's mother, Jennifer Hoffman, acknowledged that the teenager had used drugs but said she was not aware of any deeper conflict between him and her mother.

The Hoffmans were struggling to care for their daughter who has a brain tumour, when Layne offered to look after Jonathan during his final year of school in 2011.

"It's really hard to comprehend that your own mother could do something like this to your own child," Jennifer Hoffman said.

Jerome Sabbota, defending, said Layne was devastated by the verdict and contrite over her grandson's death.

"She punishes herself every day," he said. "The legal system does what the legal system does. The jury felt that it wasn't appropriate self-defence."

Layne testified that Jonathan Hoffman was loud, coarse and argumentative in the weeks leading up to the shooting. On the day of the fatal argument, he had tested positive for synthetic marijuana.

Layne said Hoffman demanded $2,000 and a car to flee Michigan. She claimed he kicked her and struck her in the face before she shot him.

Paul Walton, prosecuting, argued that she did not tell police she had been attacked but immediately confessed to the killing. A hospital nurse who examined her after her arrest said she displayed no injuries.

The most compelling piece of evidence was the victim's plea for help recorded during a desperate call to 911.

"My grandma shot me. I'm going to die. Help. I got shot again," he told the dispatcher.