What rifle bullets do:
What it will do, is punch a hole in one side, deform and mushroom to a larger size, thereby making a larger hole as it travels through tissue and doing dmage to organs. It will drag behind it a vacuem bubble that expands inside the body rending and damaging further tissue untill the velocity decreases and it catches up with the bullet and deflates. The flattened and mushrromed bullet will usually then exit leaving a larger hole than the entrance. Entrance wound of a .30 caliber rifle (.308 or 7.62 or .30/06, .30/30 7.62x39 (ak round)) will be .30 of an inch diameter. Exit may be half an inch or up to a size you could put your fist into.
People do not get there heads 'blown off', however a high velocity rifle like this with a shot to the head will do considerable damage, shattering the skull and leaving a even larger wound than a body shot, simply because the braincase/skull is full of basially a liquid matter. Liquid does not compress under pressure. With a high velcocity bullet there will be a lot of matter blown out the back. For the truely grisly, and for this I apolgise, with a head shot and a high velocity weapon the eyes may be forced out of the sockets under the pressure and sit outside the skull over the eye cavity.
This same bullet for example, if shot through an empty dixie cup balanced precariously on top of say, a coke can, will not move the cup at all. It will poke a .30 caliber hole in one side and out the other without moving it. If you shot the full coke can, because of the water content, it will explode like a handgrenade.
(While we are on the subject, brain matter is not grey. It looks a tan sort of colour. )
As I described above a rifle hit will drop someone even without hitting the spine or brain simply because of the amount of internal damage done and the shock received by organs and muscles adjacent to the wound area.
What handgun bullets do:
Handgun bullets travel at a much slower velocity and do not do the devastating damage a rifle will do. But trust me that you will still notice it right away.
A bullet shot from a pistol or revolver will not knock a determined adrenaline-fueled attacker down - unless it gets him in the right place. The right place is the nervous system. By this I mean in the neck or anywhere along the spine. This will drop him in his tracks.
A truly enraged or stressed individual may take a shot through the heart and still continue for many seconds on adrenaline before his brain starves of oxygen and he passes out. Wounds from a pistol that do not immediately incapacitate the nervous system, wills till be fatal, but may take a few seconds or minutes to cause death. A chest shot that takes out a lung but misses the spine will cuae death but not immediately. By the time he has died from it he may have killed the shooter.
This all depends on the state of mind of the victim. If he is unthreatened and unaware he is in danger he will succumb to these wounds quickly. A man in a fight may not, due to adrenaline and the determination of his character.
In general, the wounds receieved from a firearm are much more devastating than most people realise.
However, there is no firearm or bullet that can knock or blow a man off his feet. It is a matter of physics. People die because of damage done to their organs.
There is no shotgun or large caliber pistol that can throw a man accross a room with the impact of the bullet or shot. It will, however, divest him of his essential life support system in the blink of an eye.
