Bah, Max Brooks just creates a long winded explanation to make the rest of his story plausible, but it makes less sense when you actually look at real cluster munitions strikes or the damage a rotary-barrel machine gun can do, especially when you think of shooting them relatively level into a hoard. .50 caliber projectiles are big and heavy and capable of doing not just wounding damage, but structural damage to bones and possess enough energy to do this to multiple bodies at once. It only gets worse for zombies as you climb into larger munitions, many capable of fragmentation airbursts, designed for targets behind barricades or in buildings.
I think there's probably real tactical value to damaging zombies as much as possible -- a zombie missing his legs may still be capable of advancing but not like an actual zombie capable of bipedal motion. Many may actually be rendered immobile if they aren't actually killed.
And then I think of even simple mechanical machines. Look at a flail deminer -- it's basically a horizontal shaft with chains attached to heavy steel weights. It spins and pounds the ground in front of it to set off mines. Driving one of this into a hoard would simply shred the zombies. An even better effect would be the same kind of mechanism, but with the shaft horizontal and the weights spinning in the horizontal plane. Guaranteed to crush skulls.