Zombies can and do drag themselves, or even worm their way across fields. Until the brain is destroyed, they’re a threat.
Head shots are much easier when zombies are moving slower. I suspect a platoon with 8 guys shooting an M2 and the rest with scoped rifles could easily protect an encampment from thousands of zombies.
Bird shot doesn’t hurt when it falls on you, unless it gets in your eye. The recoil from the shot might be enough to cause serious neck damage.