There already IS a special tax on ammunition. Currently 11%. Started in 1934.
Excise taxes totaling more than $478 million flowed this year into U.S. wildlife conservation programs thanks to The Federal Aid in Wildlife Restoration Act of 1937 (popularly known as the Pittman-Robinson Act) and the Federal Aid in Sport Fish Restoration Act of 1950 (Dingell-Johnson Act). In 1984 a third statute, the Wallop-Breaux Act included a portion of federal fuels tax and import duties on fishing tackle and pleasure boats. Over $8.5 billion from the federal measures have funded U.S. wildlife programs since the two acts became law.
These statues are intriguing examples of sportsmen supporting wildlife and wild habitat. They are described as self-imposed taxes lobbied for by hunters, gun owners, archers, fishermen, and boaters. The Pittman-Robinson Act imposes an 11 percent excise tax on “sporting” firearms and ammunition, a ten percent tax on handguns, and 11 percent sales tax on archery equipment. Fully half of the monies collected via the handgun and archery equipment taxes goes to state hunter education and safety programs.