A MOTHER of three will spend the next 20 years in jail for firing a gun into a wall during a domestic dispute, even though no one was hurt.

Marissa Alexander had never been arrested before the US incident in Jacksonville in 2010, when she fired a single shot to scare off her husband who she felt was threatening her.

Alexander, a 31-year-old mother of a toddler and 11-year-old twins, had claimed self-defence, tried to invoke Florida's "stand your ground" law and rejected plea deals that could have won her a much shorter sentence. But a jury found her guilty as charged: aggravated assault with a deadly weapon - and Florida's mandatory-minimum gun law dictated the 20-year sentence.

At her sentencing, Alexander's relatives begged Judge James Daniel for leniency, but he said the decision was "out of my hands".

"The legislature has not given me the discretion to do what the family and many others have asked me to do," the judge said.

State Attorney Angela Corey, who also is overseeing the prosecution of shooter George Zimmerman in the Trayvon Martin case - in which the neighbourhood watch volunteer fatally shot an unarmed black teenager - stands by the handling of Alexander's case.

Ms Corey said she believed Alexander aimed the gun at the man and his two sons, and the bullet could have ricocheted and hit any of them.

Florida's "10-20-life" law was implemented in 1999 and has been credited with helping to lower the violent crime rate.

Anyone who shows a gun in the commission of certain crimes gets an automatic 10 years in prison. Fire the gun, and it's 20 years. Shoot and wound someone, and it's 25 years to life.

Critics say Alexander's case underscores the unfair sentences that can result when laws strip judges of discretion.

Greg Newburn, Florida director of the Families Against Mandatory Minimums advocacy group, said the judge should have the authority to decide an appropriate sanction after hearing all the unique circumstances of the case.

US Representative Corinne Brown, an advocate for Alexander, said: "The Florida criminal justice system has sent two clear messages today.

"One is that if women who are victims of domestic violence try to protect themselves, the 'stand your ground' law will not apply to them.

"The second message is that if you are black, the system will treat you differently."