The U.S. Army is buying 1.8 billion 5.56mm and 7.62mm bullets this year. Before September 11, 2001, the army was only buying 300-400 million a year. These are the bullets troops fire from their assault rifles and machine-guns. Most (about 80 percent) of the ammo is fired off in training. Because of the need to get non-combat troops trained to use their weapons, and use them effectively, in Iraq or Afghanistan, everyone, not just the infantry, are firing their weapons a lot more in training. Reserve troops, in particular, are firing much more ammo than they have ever done before. Normally, reservists only fire their weapons once a year, and sometimes they miss that. No more. 

The additional ammo has been obtained by, first, drawing down war reserve stocks. Taking over half a billion rounds from those stocks, plus buying even more from civilian manufacturers (in the United States, Canada, Taiwan and Israel), working round the clock, and putting mothballed production facilities to work, has kept the troops supplied. The current high production levels will remain until the war reserve stocks are rebuilt. In the meantime, training will continue to use more ammo than in the past. Over the past decade, use of live ammo in training had been allowed to decline. That has been stopped. Ammo usage in training will remain at high levels even after American troops leave Afghanistan and Iraq, at least until the lessons learned this time around are forgotten.