Well, I've only been playing this game for less then a week now so I'm no expert on heavy weapons in game terms. However heavy weapons are used in the following ways in real life military tactics:
In General: Heavy weapons are usually squad support weapons. By this we mean that they are designed to support the force at squad level. Machine guns are the most common for this purpose today. Heavy weapons have the following traits in common they all - possess above average or greater firepower, they are designed for specific applications, they usually require riflemen to support the heavy gunner, and in return support the riflemen, are usually few in number due to their weight, time required to deploy, and the limited number of tactical applications for which they excel at. Heavy weapons are vital in building a well balanced, well prepared fighting force.
Machine Gun: Machine guns are designed to kill lots of lightly armored target such as massed light infantry forces. Machine guns that fire a heavier caliber shot, such as the 50. cal M60, have sufficient power to punch through light armor such as jeeps and trucks. Machine guns are usually standard issue squad support or platoon support weapons. When machine gunners take to the field they are usually supported by one or more riflemen who are usually employed to prevent flank attacks on the gunner. A machine gun may have a crew of two (1 gunner, 1 reloader) though some modern day machine guns are usually simple and light enough to be carried and fired by one man.
Rocket Launcher: These heavy weapons are usually designed to take down heavy armor such as enemy main battle tanks. Rocket launchers usually consist of a firing tube, a internal explosive charge, and rocket. Some are one shot weapons while others are reloadable. Rocket launchers have changed significantly in the last 70 years however the basic concept is the same. Rocket launchers are bulky and heavy and can be dangerous. Usually a rocket launcher has a kill zone that exists behind the person firing it. Anyone caught in this kill zone at the time of firing will be killed by the launcher's incendiary and over pressure effects so a person firing a R. Launcher should make sure his rear is clear of friendlies before firing, if possible. Weapons of this class fire two types of munitions those that fire specially designed grenades are capped rocket propelled grenades or RPGs and those that fire a traditional rocket. RPGs are heavily used by terrorists to take down clusters of infantry, the most common RPG in use today is the stinger.
Grenade Launcher: Grenade Launchers are similar to a RPG except that they usually have a somewhat smaller explosive yield, do not require line of sight, and can usually be used many times as they tend to be reloadable. A grenade launcher is yet another support weapon unlike rocket launchers and depending on the rounds used can be effective from anything from light infantry to light or medium armor. Grenade launchers are primarily used when you need the explosive yield of a R Launcher but the versatility and angling capability of a grenade. More primitive and archaic grenade launchers fired actual grenades and consisted of a tube or explosive plate that lobbed grenades via a crimped rifle shot.
Flamethrower: Another weapon that excels in a specific situation. Flamethrowers are normally employed to kill enemies within considerable fortifications that are either too hard or well designed to be taken down with rocket launchers or grenade launchers or where either weapon simply can't reach the people inside. Sometimes you just gotta cook a dude out. A flamethrower consists of a fuel tank attached to a chemical feed tube the propellant travels up the tube and out the barrel of the "gun" somewhere along the line the chemical passes through a pilot flame and ignites creating a jet of fire that is expelled under high pressure to increase the range and consistency of the fire stream. Some flamethrowers rather then use a flame to ignite the fuel use a second chemical that when combined with the first produces a flame. Flamethrowers can also be used to create a sort of mobile no mans land while moving across a open field but if the fuel tank is hit by enemy fire it is likely the flamethrower person and anyone near him will be engulfed in a ball of flames as the unit explodes so caution must be used when deploying these weapons.
Did I miss any heavy weapons?