MIAMI—Quartavious Davis is still shocked by what happened to him in a U.S. federal court two months ago.

“My first offence, and they gave me all this time,” said Davis in an interview at the Federal Detention Center in Miami. “Might just as well say I’m dead.”

Davis was convicted of his role in a string of armed robberies in the Miami area in 2010. His accomplices testified against him, saying he carried a gun during their crimes and discharged it at a dog that chased them. But Davis was not convicted of hurting anyone physically, including the dog.

Davis would occupy no place at all in the annals of crime if not for his sentence. Now 20 years old, he was sentenced to 1,941 months in prison — 162 years and a bit — without the possibility of parole.

The U.S. Supreme Court recently decided that life sentences without parole for defendants under the age of 18 constituted “cruel and unusual punishment,” even in cases of murder. Unfortunately for Davis, he was 18 at the time of his crimes.

Nonetheless, Davis’s lawyer will argue that Davis’s sentence to die in prison also constitutes “cruel and unusual punishment” on the grounds that Davis is a “first offender,” having never before been charged with a crime.

“Just as the Supreme Court recently held that the Constitution bars taking away all discretion from judges in sentencing juveniles to life imprisonment for committing murder,” said lawyer Jacqueline Shapiro, “so also is it cruel and extreme to allow unfettered prosecutorial discretion to force a sentencing judge to impose a life sentence on a teenage first offender convicted of lesser charges.”

Davis’s unusually long sentence results from a controversial practice known as “stacking,” in which each count of an indictment is counted as a separate crime, thus transforming a first-time defendant into a “habitual criminal” subject to several sentences and mandatory sentencing guidelines.

“Any law that provides for a mandatory term of imprisonment for a 19-year-old first offender that exceeds a century has got to be unconstitutional,” said Michael Zelman, the court-appointed lawyer who represented Davis at his trial.

Zelman resigned from Davis’s case after filing a notice of appeal. If Davis’s new lawyer, Shapiro, has her way, the Supreme Court may ultimately decide the issue. The case will be appealed first to a circuit court in Atlanta.

Until then, Davis’s story will be a prominent case in point for both sides in an increasingly heated debate, pitting those who would protect society from the prospective dangers posed by serial criminals against those who see the United States — whose overcrowded prisons house fully one-quarter of all the prisoners in the world, most of them black — as a bastion of injustice.

When he was arrested on Dec. 23, 2010, Davis was an unemployed dropout living with an aunt in Goulds, a poor, predominantly black neighbourhood south of Miami. According to expert testimony at his trial, Davis suffers from a learning disability and bipolar disorder.

At the time of his arrest, he said he was living on $674 a month in Social Security disability payments and hoping to get back into school to learn a trade.

On Feb. 9 of this year he was convicted of committing seven armed robberies at fast-food restaurants, a Walgreens pharmacy and other commercial establishments in the Miami area from August to October of 2010.

Davis, who still maintains his innocence, was the only one of the six men charged who went to trial. The others cut plea deals that left them with sentences of nine to 22 years in prison. As the odd man out, Davis was convicted largely on the basis of his accomplices’ testimony.

He received seven years for the first of the firearm counts against him and 25 years apiece for each of the six subsequent counts. The law, as written by Congress, requires the sentences to be served consecutively. In prison slang, such sentences are sometimes referred to as “life on the instalment plan” or “running wild.”

During the prison interview, Davis was advised by Shapiro not to discuss many specifics about his case.

According to the trial transcript, one of Davis’s accomplices testified that Davis fired his weapon on two occasions — at the dog who chased him and 11 days later outside a Wendy’s restaurant they had just robbed. He said Davis traded gunshots with a customer at the restaurant as he and three others sped away in their getaway car.

The accounts of Davis’s firing his gun were otherwise uncorroborated.

The armed customer outside Wendy’s, school custodian Antonio Lamont Brooks, was unable to offer positive identification of the man with whom he exchanged gunfire. But he was unhurt and squeezed off enough rounds from his 9-mm handgun to leave one of Davis’s accomplices with a bullet wound in his left buttock.

It is not clear why prosecutors decided to throw the full weight of the law at Davis. (They declined to comment on any aspect of the case.)

Florida, though, has a history of “very zealous” prosecutions, according to Marc Mauer, executive director of the Washington-based Sentencing Project, which advocates for reform in the criminal-justice system.

For example, Florida leads in the number of juveniles given life sentences without parole for lesser crimes than murder — sentences the Supreme Court declared to be unconstitutional in 2010. Florida and other states are now trying to determine how to resentence or grant parole to inmates affected by that ruling.

According to a recent study by the Pew Center on the States, Florida was first, among the 35 states reporting, in increases in time served in its prisons from 1990 to 2009.

In one recent, highly controversial Florida sentencing, Marissa Alexander, an African-American woman in Jacksonville with no previous criminal record, was sentenced to 20 years for firing a pistol twice into the air while trying to ward off an attack by her abusive husband.

Denied the protection of Florida’s controversial “stand your ground” law, the 31-year-old mother of three was convicted of aggravated assault and given the mandatory sentence for anyone who fires a gun in commission of the felony.