Schuyler Pyatte Barbeau is accused of failing to pay a tax and register a firearm with the federal government and that required that the tax be paid. However, the "firearm" was not a "firearm", but separate parts, when the government received it, as explained in "The Arrest of Schuyler Barbeau". There were two barrels, a receiver, optical devices, and other items, in a case that was delivered to a Confidential Human Source (CHS), identified as Oliver Murphy.
