Yesterday, the Supreme Court decided Rosemond v. United States (No. 12-895), addressing what constitutes aiding and abetting the use or carrying of a firearm during and irelation to any crime of violence or drug trafficking crime under 18 U.S.C. § 924(c). In an opinion authored by Justice Kagan, the Court held, "[T]he Government makes its case by proving that the defendant actively participated in the underlying drug trafficking or violent crime with advance knowledge that a confederate would use or carry a gun during the crime’s commission. We also conclude that the jury instructions given below were erroneous because they failed to require that the defendant knew in advance that one of his cohorts would be armed."
For an analysis of the opinion see this SCOTUSblog post.