The justices had three choices. By the end of the arguments, most of them seemed to have settled on a middle ground.
The law imposes the mandatory sentences on people convicted of unlawfully possessing firearms if they had already committed three violent felonies or serious drug offenses. The question for the justices was how to determine which drug offenses count under the law, which refers to a schedule of controlled substances overseen by the attorney general.
That schedule is revised from time to time, giving rise to the puzzle in the case.
Depending on which version of the schedule applies, a state drug conviction may or may not count as a strike under the federal gun law. Lawyers in the two consolidated cases on Monday gave the justices three options for deciding which schedule applied: the one in force when the defendant committed the state drug offense, the one in place when the defendant committed the federal gun crime or the one that applied when the defendant was sentenced for the federal gun crime.
The first case involved Justin Brown, whose lawyer argued in favor of the third position. Several justices seemed unpersuaded.
“Why does it make sense or why would it make sense for Congress to say that two defendants who were convicted at the exact same time should be sentenced differently simply by virtue of when their sentencing happened?” Justice Amy Coney Barrett asked the lawyer, Jeffrey T. Green.
Mr. Green acknowledged that his approach would “create some arbitrariness.”
But Justice Ketanji Brown Jackson, who once served on the United States Sentencing Commission, said that approach reflected the usual practice. “A federal judge applies the sentencing law at the time of sentencing,” she said.
Mr. Brown committed, among other things, four state marijuana offenses in Pennsylvania between 2009 and 2014. At those times, as well as when he committed a federal gun crime, the drug schedule in effect made him subject to the 15-year sentence. But by the time he was sentenced for the federal crime, the schedule had changed in his favor.
A federal appeals court ruled that the middle choice — the schedule in place when he committed the federal gun crime — was the one that counted, affirming the 15-year mandatory sentence.
The middle choice would have spared the defendant in the second case. That defendant, Eugene Jackson, committed, among other things, cocaine offenses in Florida in 1998 and 2004.
By the time he committed a federal gun crime in 2017, the schedule had been changed in a way that benefited him. But a federal appeals court ruled that the schedule in effect when he committed the state drug crimes counted, affirming a 15-year sentence.
Andrew Adler, a lawyer for Mr. Brown, said the time that mattered was when his client committed the federal gun crime.
Mr. Adler started his argument with a thought experiment. Suppose, he said, that Congress had amended the gun law after a defendant had committed a state crime to drop that crime from the ones that required the mandatory sentence. No one would dispute, he said, that the amended law would apply in determining whether the state crime triggered the mandatory sentence.
The analysis did not change, Mr. Adler said, simply because a schedule was involved.
That point seemed to resonate with several justices, and they pressed a lawyer for the federal government, Austin Raynor, about it.
Mr. Raynor, who urged the justices to look to the earliest of the three possibilities — the schedule in place at the time of the state drug offenses — acknowledged that an amendment to the statute would require the middle-ground approach. But he said changes to the drug schedule were a different matter.
Justice Clarence Thomas disagreed, saying that a revision of the schedule in effect amended the statute.
Justice Sonia Sotomayor also appeared unpersuaded. “I think this is the most serious weakness in your argument,” she told Mr. Raynor, “because it doesn’t make much sense to me.”
Justice Elena Kagan called the distinction “a little bit mysterious to me.”
She added: “It seems perplexing as to why you would have a different rule than you would if Congress had just listed the substances.”