To add to this answer, here are some info on projectile penetration mechanics from The Long Guide (edited to be in bulleted list format by me):
- All guns except shotguns penetrate breakable glass. It usually becomes damaged on the first hit, but the bullet still hits people or objects behind it dealing full damage.
- Enemy weapons do not penetrate.
- Bullets penetrating shields deal only 25% of their damage, provided the weapon is capable of that (sniper rifles, shotgun slug ammunition).
- Penetration of walls and enemies does not reduce the damage.
- When a bullet impacts somewhere and the weapon is capable of penetration against glass/walls/shields/enemies, the game spawns a new projectile 0.0125 seconds later and ignores the old one. The new projectile spawns 40 cm (20 cm against shields) behind the point of impact and is then affected by spread once again (e.g. firing a sniper rifle from the hip causes the new projectile to have higher spread regardless of the trajectory of the original projectile). The new projectile has some data from the old one.
- In particular, a projectile and its subsequent spawns can never penetrate more than one wall.
- The indestructible glass on some cars counts as a wall, so while sniper rifles can penetrate a window causing a new projectile to spawn inside the car, the new projectile will hit another window or the frame on the way out and vanish.
- The type of wall or obstacle is of no consequence, as long as it is thin enough. As a consequence of the projectile spawn delay, firing a loud weapon at a cool enemy behind an obstacle alerts him first before the new projectile hits him.
Source: Weapon Stats - Remarks - The Long Guide
The armor piercing capability [of the 'Silent Killer' skill] only affects the body armor on Tans (it has no effect on Bulldozers, etc.). Without penetration, the front Tan armor fully blocks any incoming damage, so that not even hit indicators appear. With penetration, the bullet will deal full damage. Sniper rifles [and shotgun slug ammunition] have a 100% chance to penetrate and therefore do not benefit from an extra penetration chance.
Source: Remarks on Ghost skills - The Long Guide