Pistol Grips

What is it?

Pistol Grip of an AR-15

A pistol grip is essentially a type of grip that orients the shooter's hand in a vertical forward position, similar to holding a pistol.

Why are they more lethal? 

Well, the purpose of using a pistol grip on rifles such as the AR-15 or AK-47 is really to provide a comfortable grip while shooting. This style of grip arguably provides better controllability both while shooting and while maneuvering the weapon. It does not offer any direct improvement it the weapon's lethality (ammunition, magazine size and accuracy are unchanged).