It is necessary to match the gun barrel with the height of the eyes to aim accurately while shooting with the AK 47. But if the stock is not in-line with the barrel (or even if it’s straight, it is at an extreme angle) it is hard to point the muzzle straight, so the muzzle jumps up from the recoil. During sustained full-auto fire, the cumilative effects of recoil can quickly force the muzzle off-target, something that is exacerbated with a non in-line stock. In just a few shots in full-auto, the weapon becomes an anti-aircraft gun.