The unanimous decision makes it harder for courts to find lawbreakers deserve extra jail time because they used or carried machine guns during their crimes.
A federal law subjects anyone who uses or carries a "firearm" during a violent or drug-related crime to five years in prison. The term jumps to 30 years for anyone who uses or carries a "machine gun" during that same crime.
Federal appeals courts had split on whether determining use of a machine gun is an element of the offense a jury must find beyond a reasonable doubt, or merely a sentencing factor a judge gets to determine by a preponderance of the evidence.
The nation's highest court said use of a firearm must be determined by the jury.
"We believe Congress intended the firearm type-related words it used... to refer to an element of a separate, aggravated crime," Justice Stephen G. Breyer wrote for the court.
Five Davidians were convicted in 1994 in the killings of four federal agents during a botched Bureau of Alcohol, Tobacco and Firearms raid on the compound outside Waco.
The raid led to a 51-day standoff that ended when flames swept through the compound. David Koresh and some 80 followers died during the inferno, some from the fire and others from gunshot wounds.
A federal jury acquitted the five Davidians of murder and conspiracy-to-murder charges but convicted them of voluntary manslaughter. Each was sentenced to 10 years in prison for that conviction. The jury also found them guilty of using firearms.
The presiding judge tacked on 30-year sentences for four of them and a 10-year sentence for the fifth after finding that each had used a machine gun.
Renos Avraam, Brad Eugene Branch, Jaime Castillo and Kevin Whitecliff drew the 30-year sentences; Graeme Craddock the 10-year term. Craddock also was sentenced to a consecutive 10 years for using a hand grenade.
The jury never had been asked to determine what types of firearms the five had used. The judge made that determination during sentencing.
In appealing their sentences, the Davidians relied heavily on a decision in which the justices last year said carjackers cannot be given tougher sentences unless a jury, not a judge, determines that victims were seriously injured during that federal crime.
The case presumably will return to a federal trial court where new sentencing hearings will be conducted. Today's decision did not suggest appropriate new sentences.
The case is Castillo vs. U.S., 99-658.