From my Readings:
1. SUPPRESSIVE FIRE
It only serves a purpose in maneuver-type combat. Basically, when using full auto, it's not so much about actually hitting anything as it is keeping the other guy's heads down while you and/or your buddies are moving around, trying to out flank or out maneuver – or run the hell away. I was once told when I asked about the purpose of full auto that it was "expensive noise."
Full auto is for firing and maneuvering, suppressing or spraying a hidden target in hopes of getting hits.
You pretty much got it; suppressive fire is the phrase you are looking for. Same for three-round burst.
You can kill people with full auto fire, but pinning them down and keeping them under cover is what it's about. Then you get the big stuff on them, or sneak around back and let 'em have it.
In my platoons, the only time full auto fire was permitted was when making contact: either a chance unplanned contact, a meeting engagement or the initial moments of an ambush, breaking contact, final protective fires and when engaged in certain CQB tasks like clearing trenchline and buildings.
2. GREATER FIREPOWER
Raking enemy vehicles with rapid fire when single shots can't do the job.
Providing the "shotgun effect" of bullet scatter, at distances much greater than could be attained with actual shotguns.
Full-auto is also useful when being overwhelmed, for instance when reacting to a near ambush.
During MOUT, when the exact location of an enemy in a room is unknown and no friendlies or noncombatants are expected, you would enter firing full auto (after having tossed in frags first). Even 'machine guns' such as the 249, M2 or M60 are fired with bursts, and not on full auto except in certain instances. It conserves ammo, increases accuracy and extends barrel life.
I read an interesting bit on the use of full-auto, especially with MGs, in Iraq. It seems the hajjis/stinkies, when involved in a direct fire fight (meaning they're exchanging gunfire with our boys, as opposed to planting bombs or other such nastiness) have a tendency to break contact when our guys brought machineguns, especially M-240s and M-2s, into the fight. This knowledge has been used two ways:
- for convoys, getting the machineguns into the fight means you can make the enemy run away and unass the AO.
- Marines looking to kill the enemy would keep the machineguns in reserve, simply trading rifle fire until they could get artillery or air support to send the bad guys to meet Allah.
A shotgun could accomplish this same purpose, as others have pointed out. So, when you're approaching uncertain factors when you may have little time to react, the ability to put out a larger field of fire is certainly of use.
3. ET CETERA
Don't discount all soldiers as not being able to hit anything with full auto. My father is an ex Green Beret and has a full auto AR-15. I've seen him put a magazine in a paper plate at about 75 yards full auto without letting up. A few flyers but most all were nicely grouped on the plate. So with enough practice, which he had more than a little over in Vietnam during his three tours, one can learn to control full auto, and how to single-fire when on full auto.
You need to evaluate your ammo supply. If you got it to burn, then burn it! But, in most cases, you will not; so it's semi.
Three round bursts basically help assure that the intended target will go down and go down faster
I'm not sure what the numbers are for this current war overall, but there was an incident recently in Afghanistan in which two of our snipers (the area was so target rich the spotter was shooting, too) took 76 targets with 79 rounds from SPRs. Lemme see, that's 1.034 rounds per dead Taliban. I'm sure that's not the norm, but I'm also sure it levels the numbers out a little.
If the bad guys are less than seasoned troops, they will often break and run, and you win for free; if they are seasoned soldiers, they will try to concentrate on your full-auto dudes and kill them.
They can turn the intimidation around and make your dudes afraid to fire full auto. The NVA were trained to do this.
SOG teams in Vietnam found that there was a downside to using silenced weapons in a firefight. Because they were silent, they were not scaring the bad guys into backing off.
In most military conflicts, soldiers have been limited to what they carry on their body, resupply has been uncertain, and may not happen before your team is overrun.
In Granada, the SEAL team that rescued the British ambassador was down to its last mag – and they were shooting single shot with scopes. Similarly, the SF team in Gulf War One that was extracted under fire after killing about 200 Iraqi militia, was down to its last mags.
Ammo conservation is a very important part of the job!
Remember the story of the Arizona Highway Patrol trooper who faced down a guy with an Uzi: the engagement was from the door to the rear bumper or maybe a few feet more. 32 rounds of 9mm lost to 4 rounds of .357 Mag from a revolver. As reported in USA Today in the early 1990s, complete with breathless ogawd comments about the BGs being "better armed" than the cops...really?