The first bit of information to keep in mind about these is that no gun is
truly recoilless. What this term refers to is a type of firearm, ranging from small arms to
cannon, which have means of compensating for the recoil built into their design. This does
not include guns with recoil compensators such as
tank guns with
hydraulic mounts; those seek to manage the recoil rather than offset it.
There are two general ways a gun can offset recoil. The first is to use gas ejection, and the second is to use counterweighting. Let's look at each of these in turn and see how it applies to various weapon types.
Gas Ejection
In weapons of this type, the hot gases produced by burning propellant are used not only to drive the projectile but also to limit recoil. This technique is used on
small arms, as it is compatible with multiple-shot weapons. On a recoilless gun with this system (sometimes also known as a
recoilless rifle), there are
apertures in the gun barrel's sides. These apertures (usually called ports) allow gas to escape in a rearward direction as the bullet or projectile passes the port. The gas which does escape this way will exert a forward thrust on the gun by the same mechanism as a
rocket motor. Although usually this thrust will not be equal to the recoil, it will be enough to offset the impulse to a small (or large) degree.
Although there is some energy loss due to the ejection of gases, if the ports are placed far enough forward on the weapon (towards the muzzle, or even in front of it like the baffles of a suppressor or muzzle brake) then the amount of energy lost from actual acceleration of the projectile can be miminal. Gases continue to leave the gun after the projectile has exited the barrel; these can be trapped either using ports or using front deflection baffles without subtracting any energy from the projectile's shot.
One difficulty with gas ejection recoilless weapons is that the vented exhaust is quite hazardous, and typically must be vented from the barrel of the gun which is in front of the operator. Thus, extreme care needs to be taken that these vented gases do not strike the user as they stream out. The typical means of doing so has the majority of these ports near the end of the barrel, and/or along the side opposite the gunner.
Another significant problem is that of visibility. A recoilless weapon will typically make a fairly noticeable burst of light, made more so by the larger and more multidirectional pattern of exhaust gases.
The advantage of this approach, as mentioned earlier, is that the recoil compensation mechanism is passive; it does not require additional components on the projectile or in the gun itself. Therefore, for automatic or semi-automatic fire, this is the preferred design.
Counterweighting
This technique is more often utilized in single-shot, large caliber weapons. It offers the potential for complete
neutralization of the recoil, as well as avoiding the problems of hot gas. Further, it can be used to offset larger gun recoils in a smaller system, as the gas ejection required would overwhelm some smaller versions.
In this technique, a counterweight is placed at the rear of the weapon. The propellant burn not only accelerates the projectile forward, it propels the (much more massive) counterweight backwards. If engineered properly, the net thrust on the gun mechanism can be zero; a slow-moving massive counterweight offsetting a fast-moving small projectile. Further, the counterweight does not need to be solid; it can be a dispersing substance, meaning the weapon is safe to fire in an enclosed area. The Armbrust, of German design, ejects a package of light plastic strips from the rear of the gun. While this does make a cloud of ejecta, these strips are light enough that they will not harm anyone even if they bounce off a wall just behind the gun. This was explicitly designed in to allow the use of the Armbrust (an anti-armor man-portable weapon) from within prepared bunker positions.
The advantages of this approach are, as mentioned, the ability to offset a much larger charge in a much lighter gun and/or in an enclosed space, since the ejecta can be made relatively harmless; this bodes well for man-portable systems. The fact that this system is limited to single-shot (due to the need to either replace the counterweight or use a new packaged round) makes it ideal for the shoulder-fired anti-armor weapons known as LAWs (Light Anti-armor Weapon) as they are intended to be disposable.
Disadvantages include the single-shot limitation as well as a lower velocity limit; rather than reducing recoil using waste gas, this system deliberately uses the shot to propel objects in both directions, meaning the muzzle velocity of the projectile is limited by the projectile's mass as accelerated by one half of the shot's energy.
Recoilless weapons tend to be mostly either man-portable disposable launchers firing HEAT or HESH rounds, which do not depend on velocity/kinetic energy, or large-caliber rifles which do but whose charge can be thus made much larger than an uncompensated weapon's can.