The purpose of the NFA requirement that the "firearms" under the Act be registered was to keep track of who paid the tax. § 5848(a) and the Fifth Amendment privilege against self-incrimination, see United States v. Freed (1971). The government argued next that the NFA provisions may be sustained under the Commerce Clause. The plaintiffs include individuals, nonprofit associations, businesses that deal in NFA firearms, together with Texas and thirteen other states. Given the longstanding, solid anchoring of the NFA in the tax power, plaintiffs have reason to be optimistic about ultimately prevailing.