Here is the law from the Fed Firearms Regs:
16. FEDERAL EXCISE TAX ON FIREARMS AND AMMUNITION
A Federal excise tax is imposed by 26
U.S.
C. 4181 on the sale by the manufacturer, importer or producer of firearms, shells, and cartridges. The tax is 10 percent of the sale price for pistols and revolvers, 11 percent of the sale price for firearms and 11 percent of the sale price for shells and cartridges.
The excise tax attaches only to the sale of complete firearms and ammunition or firearms that, although in a knockdown condition, are complete as to all component parts.
The term firearm for excise tax purposes includes all portable weapons, such as rifles, carbines, machineguns, shotguns, and fowling pieces from which a shot, bullet, or projectile may be discharged by an explosive. The term firearm also includes pistols and revolvers. Antique firearms are also subject to the excise tax.
Shells and cartridges include any article consisting of a projectile, explosive, and container that is designed, assembled, and ready for use without further manufacture in firearms, pistols or revolvers.
Reloading of used shells or cartridges is considered manufacturing for purposes of excise tax. Sale of such shells by the reloader is subject to the excise tax. However, if the reloader merely reloads shells belonging to a customer and is paid for labor and materials, the reloading service is not a taxable sale, as long as the reloader returns the identical shells provided by the customer to that same customer. In such instances the customer is the manufacturer and would not be liable for tax if the shells are manufactured for personal use. If the customer sells reloaded shells or uses them in a business, e.g., shooting range, the customer would be liable for the tax.
Returns and Deposits
Regulations in 27 CFR Part 53 require that taxpayers incurring a tax liability on the sale or use of firearms and ammunition file excise tax returns quarterly on ATF Form 5300.26, Federal Firearms and Ammunition Excise Tax Return. In addition, taxpayers are required to make semimonthly deposits of tax on ATF Form 5300.27.
Further Information
For more detailed information regarding firearms and ammunition excise taxes, refer to ATF Publication 5300.16, Firearms or Ammunition Manufacturers and Importers Information Booklet.