The concept of a machine gun is iconic: it calls up the stereotypical action film with a shooter wildly spraying endless bullets and killing all foes with little effort. The reality of an automatic weapon is somewhat different: it requires frequent magazine changes and consistent maintenance to ensure proper operation. While many weapons carried by soldiers today are capable of firing in a repeated, automatic fashion, the intent of a machine gun varies greatly from the standard military assault rifle.
Most American soldiers carry an M4 or M16 rifle, which contains a 5.56mm round (.223 caliber). This weapon carries ammunition in a 30-round magazine. If the weapon is fired on full automatic capability, a full magazine can be burnt up in just a few seconds. Since the standard combat load for a soldier is seven magazines (210 rounds), soldiers have been trained and disciplined not to waste ammunition in this manner. Additionally, some iterations of the M4/M16 have removed the automatic option and replaced it with a 3-shot burst fire. The most effective use of ammunition, however, is to keep the weapon firing in semi-automatic mode, which lends toward the highest degree of accuracy. Thus, the distinction between a rifle and a machine gun is clear: rifles are designed to hit targets. Many nations use the popular AK-47 weapon, which is not altogether different from the M4 in function. This weapon, however, is slightly more difficult to wield with a great degree of accuracy. The round is slightly larger, and the weapon is not designed with the same precision shooting in mind. Some versions do not even utilize stocks, which keep the weapon against the shoulder and make it more stable and accurate.
Machine guns are designed for fire suppression; that is, to propel rounds downrange toward the enemy in great quantity. The desired effect is to make enemy soldiers occupying positions duck their heads in order to maneuver and gain an advantage upon the enemy. A very similar weapons system to the M4 also carried by soldiers is the M-249 SAW (Squad Automatic Weapon). The M-249 fires the exact same ammunition as the M4, and in fact has a special slot for utilizing magazines from the rifles (although it is rarely used). The most reliable option is to fire linked ammunition, which is typically available in a 200-round box. This ammunition is chained together and can be fired continually until the end of the chain is reached. The SAW does not have a semi-automatic capability, because its intent and purpose is to maintain repeated fire towards the enemy. The SAW is considered squad-portable, which means that one man can effectively wield the weapons system. With a full load of ammunition, the SAW weighs approximately 25 pounds.
True machine guns, however, are those which are not portable. They're designed to hold positions or be utilized in emplacements on vehicles. Thus, they have a stable firing platform from which other soldiers can deal a great deal of damage toward the enemy. An example is the M240B (older version M60) which is similar to the SAW in function but possesses the larger 7.62mm round and weighs about 10-15 pounds more. The true machine gun utilized by the U.S. Army, however, is the M2, which utilizes .50 caliber ammunition and is only portable in the most agonizing sense of the word (to carry it, it must be broken down and spread out among the members of a platoon, because it is extremely heavy). This is the weapon pictured. The M2 is very reliable, and can repeatedly shoot ammunition over a long period of time. Soldiers rely upon the weapon for a number of unique capabilities, including its ability to penetrate a car's engine block and stop a potential Vehicle-Borne Improvised Explosive Device (VB-IED).
A machine gun is not an invincible weapon, however. It usually has a limited field of fire, because it can only rotate so far in a given position. It must also be reloaded from time to time, and it can jam occasionally and require remedial action to put the weapon back into service. Thus, it is ineffective when alone, but it can contribute greatly to a group by offering support in the form of a high rate of fire.