Government drops charges against Kwan
David Codrea has the story, here.
Short version: Albert Kwan is a legit machine gun collector. FBI or ATF (reports are unclear) raided his house and found two combination holster/shoulder stocks for certain H&K pistols. Kwan owned a registered, full auto H&K of that type; since it was already registered as a full auto, owning or attaching a shoulder stock would be no legal problem (you don't have to register a gun twice because it is full auto and has a short barrel with shoulder stock; one registration per gun is enough). He also had a semi-auto H&K that could take the stocks, and so the gov't charged that *that* gun plus the stocks meant it was an unregistered short-barreled rifle.
Gov't also charged a semi auto M-14 copy was a full auto, but jury acquitted Kwan of that. It did convict him on the short barrel charge. Then the court granted a new trial, finding that it should have given a jury instruction requested by Kwan ... which instruction, I might add, would kill the government's case. So the government finally dropped charges.