What this tells us: 1) Westworld's guns are confusing to new and old park visitors alike; 2) they can be programmed to damage different targets in different ways. How? In 2016, speaking at a TCA panel, Nolan revealed that the bullets, and the way they're fired, are the key. "In the original films, the guns won't operate on guests, [but] we felt the guests would want to have a more visceral experience," he explained. "So when they're shot, it has an impact. They're called 'simunitions,' [aka non-lethal training ammunition]. There's a bit of an impact, a bit of a sting, so it's not entirely consequence-free for the guests."
Also, if you asked Discover Westworld's Aeden about the guns during Season 1, the Siri-like bot would respond: "Humans can be shot, but you are under no serious risk of injury or death; our guns feature low-velocity technology which feels closer to paintballs than bullets. You will either get better at ducking or grow accustomed to the impact." That last part might explain why newbs like William fall to the ground, whereas when he becomes the seasoned vet, the Man in Black, he can take hits like a champ.