Penetration Mechanics
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Aiming properly and hitting the enemy tank are only the means to an end, and that is actually damaging and eventually disabling it. And that is not automatic. Once you have hit an enemy vehicle, the game then calculates where the shot hit the enemy, at what angle you struck the armour, the effective thickness of the armour (based on the impact angle), and thus ultimately whether your shell penetrates the armour.[3]
Impact Angle
The angle at which an Armour Piercing (AP), Armour Piercing Composite Rigid (APCR), High Explosive Anti Tank (HEAT) or High Explosive (HE) shell hits the target's armour is crucial for penetrating it. The ideal impact angle is along the normal, i.e. perpendicular to the armour plate. The actual impact angle is calculated as the deviation from the normal. For this, the ballistic flight path of the shell is taken into account, which can be particularly important for artillery guns and their high arcing trajectories if you fire AP or HEAT shells with them.
If the shell hits an external module (e.g. tracks, observation device, turret rotator), impact angle is not taken into account. The exception to this rule is the gun.
Shell Normalization
The impact angle of AP and APCR shells onto a vehicles armour is normalized, i.e. adjusted towards the armour's normal axis at the point of impact.
In case of spaced armour, shells are normalized at the point of impact on the spaced armour, and if they penetrate, continue along their normalized flight path into the vehicle. Once it impacts the hull armour, normalization occurs again and the remaining penetration potential (i.e. the original penetration potential minus the effective armour thickness of the spaced armour) is used to calculate whether the shell penetrates the hull proper.
As of update 8.6, APCR shells are normalized at 2°. The normalization amount is a constant value depending on the shell; there is no randomization.
The impact angle of HEAT and HE shells is not normalized at all. Angle is used for armor line-of-sight thickness calculations, as normal.
Ricochet
If the pre-normalized impact angle of an AP or APCR shell on the target's armour exceeds 70° (85° for HEAT), a ricochet (a specific variant of a bounce) occurs regardless of its penetration value and the shell is deflected off the target without causing any damage. You may ricochet off of spaced armour as well, and even if you penetrate that your shell may still ricochet off the underlying hull armour.
As mentioned above, impact angle is not taken into account when hitting external modules except the gun, so a ricochet off those is impossible.
A ricochet off terrain features, buildings or wrecks is impossible.
Overmatch
If the AP or APCR shell's caliber is more than 2 times the nominal thickness of the armour (Such as a 130mm shell hitting a 60mm thick plate), projectile shell normalization is increased by the following formula: basic normalization * 1.4 * shell caliber / nominal armour thickness. Note that the shell is still capable of bouncing if it strikes the armor at an angle of 70° or more from normal.
If the AP or APCR shell caliber is more than 3 times the nominal thickness of the armour (such as a 130mm shell hitting a 40mm thick plate), no ricochet will happen even if the impact angle is more than 70° from normal. The increased shell normalization described above will also occur.
In cases involving HE shells or external module hits, overmatch does not occur.
Effective Armour Thickness
Your tank is armoured with plates of varying thicknesses. The game only provides you with the nominal armour strength of the three main armour plates of your tank's hull and turret, respectively. However, the tanks are actually modeled in much greater detail. The penetration indicator can help you discover the actual nominal armour thickness of your target.
However, the nominal thickness of an armour plate is just the minimal amount of armour a shell impacting it must penetrate. As soon as the impact angle deviates from the normal, i.e. is not perfectly perpendicular to the armour plate's surface, the effective armour thickness that the shell needs to penetrate will be higher than the nominal armour thickness:
Effective_armour_thickness.png
The effective armour thickness is calculated by dividing the nominal armour thickness with the cosine of the nominal impact angle. For example, in the diagram above we have a nominal armour thickness of 100mm and an impact angle of 30°, thus we have an effective armour thickness of 100mm/cos(30°) = 115.47mm that the shell needs to be able to penetrate. In other words, at an impact angle of 30° the armour is effectively over 115.47% stronger than its nominal value.
The following table provides the coefficients for a number of normalized impact angles:
Impact Angle Effective Armour Thickness
0° 100%
10° 101.54%
20° 106.42%
30° 115.47%
40° 130.54%