This is frequently overlooked aspect of firefights. Anyone who has fired a weapon can attest to how quickly a magazine is emptied. When a weapon runs out of ammo it is useless until it is reloaded. Empty weapons don't kill people unless they are used as clubs and if the other guy still has ammo you are in big trouble.
Back in the 'old' days when weapons didn't fire as fast (bolt action rifles) it wasn't so easy to run out of ammo. With semi-automatic weapons that only take a trigger pull it is much easier. Automatic weapons are even worse. In the heat of combat all a person has to do is clamp down on that trigger. This is one of the major reasons the M16-A2 currently issued to US combat troops does not have an 'automatic' feature like it did in Vietnam.
By taking the 'automatic' setting off the 'new' M16 a frightened man has to consciously release the trigger, giving him time to reevaluate the situation and target however so briefly. It forces the soldier to think for a second and this can be important.
When dealing with civilians automatic weapons are not a good thing to have. Automatic weapons 'spray' an area with bullets and are not very accurate. Bullets fired from a machine gun are just as likely to hit innocent women and children as they are to hit an enemy combatant. This is a major reason police are not usually issued automatic weapons (among other reasons).
In order to avoid running out of ammunition at the wrong time in a firefight, fire discipline is critical. Fire discipline is firing the least amount of bullets to get the job done. Any and all weapons should exercise fire discipline.
There are five recognized rates of fire and troops should be well drilled in their use and application.
Deliberate: Five rounds per minute for rifles. Best used at long range, against small targets or indistinct targets. This should be very accurate fire.
Snap Shooting: Two rounds per exposure for assault rifles (three to five rounds for machine guns). Best used at short ranges with short exposures, like in an urban environment. This is derived from the double tap.
Rapid Fire: Twenty rounds per minute (the sustained rate of fire for the machine gun). This is best used for or against assaults since it is quick accurate fire. This should be the rate of fire used to gain or if necessary, maintain fire superiority. A standard magazine has thirty rounds so it is roughly one shot fired every three seconds.
Intense Fire: Thirty to forty rounds a minute. This is used to defeat Rapid Fire and gain fire superiority. Weapon fire should still be aimed when possible.
Full Auto: Basically everything you have as quickly as possible. Machine guns should fire eight to ten round bursts. This is used to break out of an ambush and gain immediate fire superiority.
Regardless of which Rate of Fire is used, machine guns should fire three to five round bursts whenever possible, eight to ten round bursts at the most. This gives the gunner time to readjust his aim and helps keep the barrel from overheating as quickly. It should be noted that the more rounds are fired the hotter the barrel will become, machine gun OR assault rifle. Machine guns of all calibers have been known to melt.
A high rate of fire also increases the possibility of a 'cook off' or a weapon jam. A cook off is when the powder is ignited incorrectly and does not propel the bullet completely out of the barrel. When this happens the next bullet fired turns the rifle into a low yield grenade.
It should be noted that combat troops do not throw away magazines like in the movies. Frequently magazines are hand loaded and troops will frequently carry additional cases of ammo to reload them.
The middle of a firefight is the wrong time to be reloading magazines or changing magazines for that matter.
One problem encountered is the shooter frequently loses track of how many rounds are left in the magazine. The professional will change magazines as soon as the situation permits, even if the magazine isn't completely empty. One trick is to put three tracers right before the last two or three rounds. When the shooter sees three tracers go 'downrange' he knows to prepare to reload.
Before a person 'assaults' a target at close range the pro will reload, even if the magazine isn't empty. The amateur will conduct the assault and find himself with an unloaded weapon when he needs it most.
Another advantage of reloading before you need to is the fact you still have a round in the chamber and there are fewer moves. For instance. When you reload an M-16 the bolt locks to the rear which helps the shooter realize his magazine is empty. After removing the previous magazine and inserting the new one you still have to release the bolt before you can fire (a simple operation really). The AK-47 and other similar weapons have the bolt go home. This can be confusing for the shooter because it could indicate a jam or empty magazine. After removing the magazine and attaching the new one the AK gunner has to pull the charging handle to the rear in order to put a round in the chamber. This method is much slower than an M16 and in the confusion can be easier to fumble.
If you reload before you need to then you don't have to worry about the charging handle or the bolt's position.
In most cases a machine gun can be reloaded by an assistant gunner who can attach the next belt to the one currently in the gun. However, if the machine gun does run out the top 'feed cover' has to be lifted, a new belt placed in the mechanism and the feed tray closed. Then the charging handle has to be pulled to the rear before the gunner can fire.
Some machine guns (especially newer ones) have a kind of magazine that is often little more than a box of linked ammo that is attached underneath the weapon. This box can be replaced relatively easily but the belt of ammo still has to be fed into the weapon.
Counting ammo fired from a machine gun while under fire is not practical so the gunner must frequently check to see how much he has left. A fool like Rambo who lets the belt hang loose will quickly find that the dirt, mud, blood and grime that gets on that belt of ammo will jam the machine gun. The belt of ammo might also snag on branches or other things and trip up the gunner.
Grenades, grenade launchers and rockets are used as needed. Underslung grenade launchers should be kept loaded and reloaded after they are fired. Grenades launched from the muzzle should be loaded only when required.
Carry too much ammo and you are going to be a nice slow moving target. Carry too little ammo and you might be a fast moving target. Remember the ammo has to go somewhere and the weight adds up fast. Ammunition isn't made of feathers, it is made of steel, brass and lead. Magazines are also made of metal. Magazine pouches should be located on the sides where they don't interfere with the soldier when he tries to become one with the ground while bullets zip by overhead.
It should be noted that shooting most rifle grenades does not make them blow up but it might stop or deflect a bullet.
Fire discipline is frequently the difference between resisting an assault and getting over run by one. Fire all your ammo in the first few minutes of a firefight and you can bet the enemy will still have some left to finish you off. Run out of ammo during an assault and you can bet you are going to fail.
Fire discipline separates the pros from the amateurs, or more appropriately, the living from the dead.