You only need 1 bullet to kill a zombie, a single action rifle for longer ranges and a pistol with a large clip for closer ranges. Your not trying to shoot through body armor, if you are in a situation where you need to mow down a large number of zombies you are probably dead anyways.

Its also better to use guns that have similar ammo types.

And as for a flame thrower? The only thing thats going to happen with using that is now you are going to have a zombie thats on fire chasing you.