Also... zombies! Thousands of them!
Let's say you were sitting on hundreds of thousands of rounds of .223, and had an AR set up exactly how you would like.
How many people would you need to wipe out 5000 zombies from rifle range as they marched towards you at 1 mile per hour? At what distance would you start shooting at them?
If it's just me all by my lonesome self, I could place a headshot every shot from 300 yards and closer, firing one shot every 2 seconds, with a fixed 4x scope of some sort.
Zombies moving at 1mph will close 300 yards in about 1/5 of an hour, or 12 minutes. A mass of zombies 5000 deep where each zombie consumes 4 square feet will have a surface area of 20,000 square feet. If in the rough shape of a hybrid squarish-circle mob, the radius would be about 80 feet and the width of the wall moving towards you would be about 160 feet.
12 minutes of constant shooting, 30 headshots per minute, yields 360 dead zombies. All the zombies shot would be on the leading edge of the mass, hopefully the shooter is just moving from left to right or right to left, typewriter style. 160 feet of surface area at 2 feet per zombie results in 80 zombies per wave, so I would get rid of 4-5 waves of zombies before the whole mass closed 300 yards. The 5th wave would be 10 feet away after shooting for 12 minutes.
If I had a fire team consisting of 10 total shooters, all capable of head shots at 300 yards and closer at a rate of 1 shot per 2 seconds, I'd have 3600 dead zombies after 12 minutes. At 80 zombies per wave, we'd end up chewing through 45 waves of zombies, buying us an extra 90 feet after 12 minutes of constant firing. This also leaves our flanks and rear exposed to stray zeds who are not part of the main herd.
Fifteen shooters dedicated to herd destruction shooting at that skill and rate would yield 4800 dead zombies after 12 minutes. They would push through 4800 of the 5000 zombies in 12 minutes, 60 waves which would yield 120 feet of distance for the remaining 200 zombies. If zed is moving 1mph or 300 yards in 12 minutes, then he will need 96 seconds to cross 40 yards. Fifteen marksmen can fire 200 shots in 90 seconds with no problem. That just leaves flank and rearguards. Add 2 shooters to the left and right sides to sweep up strays that would otherwise distract the main 15 shooters, and 2 to the rear. 21 shooters with a 500 round loadout should be able to eliminate that herd.