All hands, stand by for another Wall of Text!
All tank guns in World of Tanks are breech-loading rifles, so no need to discuss smoothbores, muzzle-loaders or any other variants. The four statistics you are most worried about are accuracy, penetration, rate of fire, and damage caused.
A number of factors determine the accuracy of a gun. Primarily, these are the construction of the gun itself, the design of the cartridge, and the fire control system. Variances or inaccuracies in any of these will result in a decrease in accuracy. No gun as yet built is ‘perfectly’ accurate. The level of inaccuracy inherent in a typical gun is measured in either mils or minutes of angle. The latter is more familiar to civilian rifle shooters, AFV crewmen will be far more familiar with mils. The accuracy figure for a gun in World of Tanks is measured in milsx10.
A mil is effectively the angle described by an item one meter in length when viewed at a distance of 1,000 meters. (or 1 foot at 1,000 feet). Mathematically there are 2000x Pi mils in a circle (about 6,283). For ease of mental arithmetic, NATO militaries round this off to 6,400, though not all countries do so. Modern tank sights are equipped with mil scales, so that if you know the size of a target, you can determine how far away it is. For example, a 2m high tank at 1,000m will appear 2 mils high. At 2,000m, it will appear 1 mil high, and so on. Of course, this is more an academic issue, as range-finding is automatic in World of Tanks.
More importantly, the figure for accuracy will tell you the chances of hitting a target at a given range. As the figure for accuracy is in tens of mils, a gun with an accuracy figure of 0.35 will place its shot within a circle 3.5m in radius at 1,000m. Or, if you wish, a circle 3.5m wide at 500m. This is significant. For example, a King Tiger is only 3.1m high. It is quite possible for a ‘perfectly aimed’ shot with a 0.35 accuracy gun to miss a King Tiger at 500m. It probably won’t, as the dispersion is on a Gaussian curve, but a ‘miss’ would not necessarily mean that you had done anything wrong.
Factors of the gun which affect accuracy are barrel length, stability, temperature distribution and simple production quality control. Generally speaking, a longer barrel will result in a more accurate gun for two reasons. Firstly, the projectile spends more time being affected by the rifling, thus imparting better spin. Secondly, as the projectile spends more time in the barrel, it thus means that it has the propellant effects of gasses acting upon it for a longer time, meaning greater acceleration time and thus a faster projectile. Faster projectiles gain the benefits of a shorter time of flight, meaning that less lead is required to hit moving targets, and there is less time for external factors such as crosswinds to take an effect. They also result in a flatter trajectory, allowing some error in range calculations. In real-world terms, this latter allows for a longer ‘battlesight’ range: Battlesight is a default range which is used when no range information is available: It is a range setting at which, up to a certain distance (about 1.3x battlesight range) a shell is effectively guaranteed never to be too high in its arc of flight to hit a target. Although range calculations are, as mentioned, automatic in World of Tanks, arc of flight can by important especially when leading at targets, as a slow round will fly a higher arc in order to hit the point of ground your sight is aimed at which is in front of, and beyond, your target tank. So why aren’t all cannons as long as possible? Several reasons. Firstly, they just get heavy and droop, bend and flex. Especially large caliber ones. Secondly, you reach a point of diminishing returns, as the longer that the projectile is in the barrel, the more time the friction of the rifling has to impart drag. Further, if it’s really overdone, the propellant gasses stop expanding, and a vacuum is created behind the projectile, further decelerating it.
Then you have the factor of the round fired: Just how it is designed, the aerodynamic efficiency or balance. Big fat rounds like a 152mm HE are going to have pretty major drag issues, reducing its effective range even further than its already short distance caused by the usually short barrels its fired out of. The other extreme is also possible, the APDS round fired by the 17-Pounder was notorious for its inaccuracy.
Fire control systems are probably the least critical for our purposes: Variances in the quality of the optics and the reticles can have an effect, but there’s not much to be done about it as a player in World of Tanks. Spotting range and aiming times are a general mechanic to reflect overall abilities in target acquisition, and target engagement, two entirely separate issues both of which can be affected by a tank's optics. It's possible to have excellent engagement capabilities while having terrible acquisition abilities, as a visit to the gunner's seat of a Panther would show.
There are two methods of traditionally penetrating armour. Good, old-fashioned, brute force, and chemical effects (i.e. explosions).
Most AP rounds work on the brute force principle: A metal ‘arrow’, piercing the skin of the target by sheer inertia (mass by velocity). Shaped charge (HEAT rounds particularly) produce this effect by way of concentrating the force of an explosion into a focused area, then punching through the armour at that point. Regular High Explosive rounds have limited ability to punch through armour, and just blow up, more usually causing structural or component damage, and knocking the crew inside about a fair bit. HESH/HEP doesn't try to penetrate at all, and just relies on transmitted vibration.
The ability to penetrate armour in the game is represented in terms of millimeters of armour at perpendicular impact. This is a variable figure, actual penetration of any particular round fired may be up or down a bit by up to 25%. As an AP round has velocity as an important factor in its penetrating capability, its ability to punch through armour will decrease the further away a target is. This is due to the air resistance acting on the projectile slowing it down over time. Chemical effect rounds such as HE or shaped charge retain their effectiveness over distance.
After it penetrates, the next question is ‘how much damage is caused’? This is where WoT deviates somewhat from real life: After all, real tanks don’t have damage points. However, the general gist of it is right to some extent. A tank may well survive a penetration by a small round better than a large one. In WWII, most AP shells had an explosive content: After impact on the armour, a delayed fuse would set off a small explosive charge which, in theory, would detonate after the round had passed through the armour plate, causing a significant emotional event inside the tank. The larger the caliber of the round, the greater the explosive payload within the projectile, with obvious reprecussions. Later munitions had no explosive content at all, and relied simply on the projectile bringing particles of metal from the armour (a process known as spalling) inside the tank with it, which would then fly about with generally unpleasant consequences. This made them more effective at penetrating armour, but does lead to the problem of over-penetration: If there’s not much armour being penetrated, the round may simply pass all the way through the vehicle and if it doesn’t hit anything vital on the way, just leave two ventilation holes with no other significant effect due to not having brought many armour particles with it on its way through.
Finally, rate of fire. A high rate of fire means that you can generally afford to take snapshots. If you miss, you’ll have another round in a couple of seconds. It also means that if you do hit and cause damage, you can do it again in pretty short order. However, generally speaking, the larger the round, the slower the rate of fire. Anything over 120mm is going to be very slow as 120mm is about the maximum size that a human loader can handle unassisted. A larger caliber gun is liable to give you a very sizeable amount of damage when it hits, but after it’s fired, it’s not going to be a threat to you again for a moderate amount of time. On the other hand, if a target is fleeting, it may well be better to hit him with a big gun once, instead of hoping that the target will hang around long enough for two or three shots with a smaller, faster gun.
A historical note on stabilisation systems, not really related to the game at all.
Gun stabilization systems first came into common usage in the interwar period on warships. They generally use the principle of ‘rigidity in space’, and the fact that spinning objects tend to resist movement.
As the mount in which the gyroscope was positioned moved up or down due to the motions of the ship, the system would recognize the pressure of the gyroscope resisting this movement, and as a result apply the signal to the gun elevation and traverse drives to attempt to keep the guns stable and pointed at the target. The reason this came into effect for warships over tanks first is twofold. Firstly, there is a lot more room on a ship for mechanisms, and secondly, a battleship’s movement due to the sea is substantially slower than that of a tank moving cross-country.
Eventually, however, the systems were miniaturized to such an extent that they could be placed into tanks. Early stabilization systems were mounted in American M3 tanks, both the light tank’s 37mm and the medium tank’s 75mm. They eventually became standard on many subsequent tanks, though M26 and M103 were exceptions. These early stabilization systems were, of course, limited. They applied solely to elevation, and, frankly, couldn’t usually keep up with the movements of the tank itself. However, they did at least tend to keep the target within the field of vision of the gunner and allow for a faster lay onto the target.
Centurion was the first production tank to have two-axis stabilization, in both elevation and traverse. Again, however, it wasn’t truly enough to allow the tank to have a proper fire-on-the-move capability, but at least over lightly rolling ground against near targets, it was good enough.
A change in the concept of gun stabilization came with the idea that the sights and the gun need not be linked. In today’s tanks, the sight is fully stabilized, the gun just tries to keep up. It is much easier for the motors to stabilize the 5”x6” mirror in a sight head than it is to stabilize a 2-ton cannon. In a normally functioning modern tank, the gunner controls the sight, not the gun. When the gunner fires the main gun, he is actually closing part of a firing circuit. There can be a small delay between the pulling of the trigger and the actual detonating of the propellant while the fire control system waits for the gun to catch up with the sight. When the sight and gun are in total alignment, the rest of the firing circuit is closed, and the round is fired. In the event of a stabilization system malfunction, the gunner reverts to controlling the gun, with the sight trying to keep up. This is why you’ll hear a description of the difference of ‘normal mode’ and ‘emergency mode’ in an Abrams tank as “In normal mode, the gun is slaved to the sights, in emergency mode, the sights are slaved to the gun”
Of course, that’s not the end of the story. There is still a further limitation of the ride of the tank. Though the sight and gun may be somewhat stabilized even over the roughest ground, you still have the problem that there is nothing stabilizing the gunner’s seat. As the tank is being thrown around by the bumps, so is the gunner. As he’s holding onto the gunner’s handles, the effect then is that (even if he can keep his eye to the sight), a lot of unintended inputs will be placed into the fire control system. There have been moves to try to minimize this effect. Obviously a smoother suspension system is a good first step, but the latest British tank, Challenger 2, has a gunner’s control handle which is fixed in position, and the gun is moved with a thumb switch akin to a joystick’s hat. As a result, the gunner can hold on to something solid as he’s being thrown around, and should be able to keep the gun on target more accurately. As a result, though claims of accuracy at 30mph on a par with stationary fire for modern tanks may be correct on the test range on roads and trails, such claims must be treated as somewhat suspect when dealing with such speeds on rough terrain.
The most recent form of gyroscopes are not really gyros at all, as they have no moving parts. Laser Ring Gyros work on the basis of a laser beam bounced off a number of mirrors, and as the mounting moves, the light projected is bounced in different angles which is registered by the system and interpreted as motion.
Vertical stabilisers in World of Tanks which can be mounted on a number of vehicles are best considered as equivalents to the stabilisers as found in American WWII vehicles. You will be disappointed with the results if you’re attempting to fire at any distance when driving at full tilt, but the speed it takes for the aiming circle to shrink to full accuracy will be increased, thus allowing you to get the first accurate shot off compared to an opponent not so equipped.