"Next Wednesday, March 30, the Court will hear argument in yet another criminal case in which the unexpected passing of Antonin Scalia on February 13 will leave an unanswered 'hole' in the Court’s deliberations. Last June, Justice Scalia wrote the opinion in Johnson v. United States, in which, after an eight-year campaign originating in Justice Scalia dissents, a majority declared the 'residual clause' of a federal repeat-offender statute unconstitutionally vague.
The question quickly arose whether that ruling should be applied to federal cases on collateral review, even though they were 'final' before Johnson was decided. That is, should Johnson apply 'retroactively'?.... Under the Armed Career Criminal Act... if a federal unlawful-gun-possession defendant has three prior 'violent felony' convictions, his sentence is increased from a possible ten-year maximum to a mandatory minimum of fifteen years to life in prison. In Johnson, a majority declared unconstitutional, as too vague, a 'catch all' part of the definition of violent felony, called the 'residual clause.' Justice Scalia’s opinion for the five-to-four majority was quite scathing in critiquing the statutory language, while the dissenters felt it extended the due-process concept of unconstitutional vagueness too far."
Writes Rory Little at SCOTUSblog. Little does a good job of explaining the question of which newly announced rules of constitutional law can be used by prisoners who have been convicted and are past the point of direct appeal. You may think it's just not fair that some people are in federal prison, for gun possession crimes, and are serving 15 years to life, when the Supreme Court has said it's unconstitutional to use the ACCA to give more than a 10-year sentence. But the analysis has to do with whether the newly announced rule is considered "substantive" or "procedural."
Missing Scalia has to do with a dissenting opinion he wrote last January in Montgomery v. Louisiana, in which the 6-3 majority, which looked at the new rule that barred automatic life-without-parole sentences for juveniles and said that it was retroactive. Scalia's dissent in Montgomery as vigorous but "Justice Scalia’s opinion finding the residual clause unconstitutional in Johnson was also vigorous."