The puzzle of 18 U.S.C. § 924(c)

The en banc Tenth Circuit has handed down a very interesting opinion on federal criminal law, United States v. Rentz. The majority opinion by Judge Gorsuch is well worth a read. It begins:

Few statutes have proven as enigmatic as 18 U.S.C. § 924(c). Everyone knows that, generally speaking, the statute imposes heightened penalties on those who use guns to commit violent crimes or drug offenses. But the details are full of devils. Originally passed in 1968, today the statute says that "any person who, during and in relation to any crime of violence or drug trafficking crime . . . uses or carries a firearm, or who, in furtherance of any such crime, possesses a firearm, shall, in addition to the punishment provided for such crime . . . be sentenced to a term of imprisonment of not less than 5 years." 18 U.S.C. § 924(c)(1)(A). That bramble of prepositional phrases may excite the grammar teacher but it's certainly kept the federal courts busy. What does it mean to "use" a gun "during and in relation to" a drug trafficking offense? The question rattled around for years until Bailey v. United States, 516 U.S. 137 (1995), and even now isn't fully resolved. What does and doesn't qualify as a "crime of violence"? The better part of five decades after the statute's enactment and courts are still struggling to say. Cf. United States v. Castleman, 134 S. Ct. 1405 (2014); United States v. Serafin, 562 F.3d 1105, 1110-14 (10th Cir. 2009). And then there's the question posed by this case: What is the statute's proper unit of prosecution? The parties before us agree that Philbert Rentz "used" a gun only once but did so "during and in relation to" two separate "crimes of violence" - by firing a single shot that hit and injured one victim but then managed to strike and kill another. In circumstances like these, does the statute permit the government to charge one violation or two?