From Gun Owners of America:

Under provisions of the Pittman-Robertson Act (P-R), handguns are taxed at the confiscatory rate of 10 percent and ammunition, rifles and shotguns are taxed at an even more outrageous 11 percent. Enacted in 1937, P-R was originally levied at a rate of 11 percent on rifles, shotguns and all types of ammunition and in 1970, handguns were added and taxed at 10 percent.[1] Many gun owners are unaware they are even paying this tax because it is collected by manufacturers, based on the wholesale price of a firearm or ammunition. Don’t be fooled, it is built into the price you pay for a new firearm or ammunition; the manufacturers are not paying the gun tax, you are.