There is hereby imposed upon the sale by the manufacturer, producer, or importer of the following articles a tax equivalent to the specified percent of the price for which so sold:
Articles taxable at 10 percent-
Pistols.
Revolvers.
Articles taxable at 11 percent-
Firearms (other than pistols and revolvers).
Shells, and cartridges.
26 U.S.C. § 4181