One of the more important mechanics in WoT, penetration. Without this thing, the game would make no sense because teir 1's would be penning teir 10 heavies. Penetration relies on angles, thickness and raw penetration of the gun. The actual penetration deviates 25% +/- from the listed penetration. So if the average pen lists 100, the actual pen would be 75-125, this also aplies for damage. There is also the chance where the shell penetrates no armor at all, resulting in no penetration no matter the amount of enemy armor, this is why you will sometimes bounce off light tanks that your gun shouldve torn right through. There are 5 type of ammunition: AP(Armor piercing), APCR(Armor piercing composite rigid), HEAT(High explosive anti-tank), HE(High explosive), and HESH(High explosive squash-head), AP is the standard ammo for most, HEAT is a ammo that doesnt suffer loss of pen over distances, but doesnt benefit from normalization rules, APCR is high velocity/pen AP, with less normalization, HE are explosives and will detonate wether it pens or not, HESH wirks more like a high-pen HE. The angle of the point of contact is also a factor of armor penetration. The greater the angle, the thicker the armor. If the angle reaches over 70 degrees, the shell will automatically ricochet, and do no damage. But if the caliber of the shell exceeds the thickness of the armor by 2 times and the penetraion is enough, then you can 'overmatch' the armor and have a chance of penning the armor at extreme angles(Ex: 50mm of armor can be overmatched by 100+mm guns). There is a normalization in AP, the standdard ammunition most tanks use, that helps reduce the angle of the tank by about 3-4 degree. There are 2 types of armor you can be shooting at, Solid Armor, or Spaced Armor. Solid armor is armor that, if penned, will cause the tank to be damaged. Spaced armor, usually tracks or the gun mantlet, can be penetrated but no damage will be taken by the tank. Spaced armor usually has another Solid Armor layer underneath, so its usually very hard to penetrate compared to hitting regular armor.