In 1937 sportsmen lobbied congress to create a 10% excise tax on guns and ammo to aid wildlife conservation. It passed and was known as the Pittman-Robertson Act, named after the two sponsors of the legislation (TRIVIA: the "Robertson" named was Rev. Pat Robertson's father). In 1970, the excise tax was expanded to include handguns and archery equipment. Since 1937, P-R has raised $4.4 billion for wildlife conservation.
So whenever you go buy a box of shells, or a brick of cartridges, you not only pay sales tax, you also pay an additional 10% excise tax into P-R.
Putting still another excise tax on ammo is on its face an absurd notion to most people and that is why it really has no hope of passing. The idea is usually raised by a Democrat congress-critter that doesn't know about P-R (hello Sen. Obama). There is also a "nuance." Democrat constituencies like animal rights and environmentalist groups, really do not want the attention brought to the fact that sportsmen voluntarily pay for wildlife conservation, that a debate over this legislation would bring.
Unless the Democrats win a longstanding super-majority indicating a mandate for more liberalism, I just don't see it happening.