Fire directed at the enemy to keep him
from seeing, tracking, or firing at the target is
suppressive fire. It can be direct or indirect
fire. Smoke placed on the enemy to keep him
from seeing targets is also suppressive fire.
U.S. Army Field Manual FM 21-75, "Combat Skills of the Soldier," Appendix G, p.16
Washington, DC: Department of the Army, 1984.
"PUT OUT ROUNDS!"
-Pvt. Joker, Full Metal Jacket
In The Beginning, in other words 'round the beginning times of the projectile weapon, ammunition was scarce. It usually represented the allocation of scant resources and/or much labor (do you know how hard it is to chip down a stone until it's round enough to fly straight over long distances? No? Try it). Even when the actual ammunition itself became cheaper due to improved projectile designs, when in combat it was still quite expensive to fire; if not in terms of materièl then in terms of time. Reloading a bow, or a sling, or (heaven forfend) a Springfield musket took time, and as Napoleon said, "Ask me for anything but time." Projectile weapon fire was just darn expensive in all the currencies that mattered in combat due to either its material cost or its time to produce. As a result, projectile fire was overwhelmingly aimed fire, intended to hit the enemy. That, after all, was the point of combat, right? Hit the other guy?
Well, along came this thing called the industrial revolution. One of the side-effects of this massive lurching change in warfighting technology was that suddenly projectile ammunition was not only cheap enough to produce that everybody could have some, it was (due to those things called machine guns) really really quick to fire, too. Now, the equation had flipped - projectiles were cheap both in terms of materièl costs as well as cheap in terms of time to fire.
The problem was that this was true on both sides.
What this meant was that everybody developed an amazing proclivity for protecting their ass, a.k.a. 'finding cover.' It became harder and harder to hit those dastardly enemies, because they were crouching or huddling behind earth, wood, metal, dead bodies, stone, what-have-you. Also, from that position, they could stick their heads up just enough to try to hit you with aimed fire when your brave lads tried to close the range, or even do something like commute to the cook tent for afternoon tea.
What to do?
The answer was suppressive fire - projectile weapon fire at both the man-portable (rifle), mounted (machine gun) and battery (artillery) levels. This was, as the Field Manual above states, projectiles fired for the express purpose not of hitting the enemy (although nobody would complain if that happened) but to force him to keep his head and other valuable bits down behind solid cover, from whence it was very very difficult for him to direct aimed fire at your troops.
The quantity of ammunition thus expended is huge. Nobody really knows how big. But it keeps going up, especially as infantry weapons which fire in automatic or burst mode become more common and large-nation wars (i.e. well-supplied ones) continue to burn. Testimony before the U.S. House Armed Services Committee on June 24, 2004 estimates that in mid-2004, for U.S. forces in Iraq alone, "[c]urrent expenditures in Iraq are reported as 5.5 million rounds per month." That works out to roughly 183,333 small arms rounds per day (pistols, rifles, small mounted machine guns) in anger. "In anger" means fired during the course of hostilities as opposed to in training operations. How many targets were hit during that time?
U.S. News and World Report tells us in late September, 2007 that "4,882 insurgents" had been killed in Iraq so far that year. Let's assume it took a month to assemble and write up those figures; that means over the first 8 months, we saw 4,882 enemy deaths. That makes approximately 610 combat deaths per month.
Overlaying the two, we find that we're expending at a minimum 5,500,000 rounds per month / 610 enemy deaths per month = ~9,016 bullets fired per enemy killed. And I'm willing to bet that that's really, really low compared to, say, World War II. But it's not a small number.
Now, of course, that's not the same as enemies hit. But bear in mind the large numbers of enemy engaged not with small arms fire but with artillery or mortars, or by the now-infamous UAVs and aircraft - those unfortunates are included in our total.
The point, then, is that we're spending an awful lot of bullets. Why? Your friend, my friend, and especially the soldier's friend, suppressive fire.
For rootbeer277, who asked.