Though nearly 70% of zombie-related deaths are due to being caught off guard, as much as half the remaining are caused by people finding out too late that they cannot kill the zombie whether it be due to lack of knowledge or ability (the remaining deaths are caused due to "complications" that an outbreak can cause). There is no real good preparation for this, the best I can do is just give you some pointers.
How-To: Kill the Zombies
Head. Head. Head.
The largest rookie mistake is by far and away going for body shots or trying to lame a zombie. THIS WILL DO YOU NO GOOD. The only method is to destroy their brain or to cut off their head. This adds a challenge to pulling off an effective shot against the undead, but it is possible.
Remember, to kill the undead, shoot them in the head!
Give and Take
Effective headshots are a matter of given and take. If you stay too far back, you increase your chances of wasted ammunition. If you get too close, then you will be vunerable while taking aim. You will probably want to be about 3/4s your effective range maximum. You will miss a shot from time to time but it keeps you safe and gives you a good ratio of hits to misses.
Eschewing Guns
Of course, if the outbreak is prolonged, a gun will be nearly a hindrance. You will have to scramble to keep ammunition and unless you are a natural shot then you will have to pause and aim to get the headshots in. A possible alternative is to use a good sharp blade: a scythe or a katana.
This helps to fix the issues of ammunition, but means you will have to get close. You will also have to keep your blade sharp and your fighting arm stands to get tired.
One other thing in favor of using a blade over a gun is the fact that you can keep the carnage to a minimum as long as you stay in control.Zombies are going to come to the smell and the noise of gunfire. With a blade, you will have less stragglers show up last minute.
Hit and Run
A good rule to keep in mind is that zombies are slow and there will always be more. If you can get by without killing the, you might be better off just knocking them back and leaving the area. Studies have shown that zombies can track you, so you will need to get at least a moderate headstart.
Zombies cannot be lamed due to pain. You will need to physically hinder their ability to move. Good, consistent shots into the knee caps will keep them from being able to stand upright, but hitting below the knee will do little good. This is one area where blades can do more for you than guns, so even if blades are not your primary weapon, they might make a good back up weapon for these special occasions.
With any luck, you will never have to use these techniques, but if they occur, remember a calm shot is better than a franctic one. Try and not be surrounded and keep an escape route in mind. They will keep coming over and over, so your main goal is to get into a well guarded area, not to kill every single one of them.
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