Humans can get shot
Key moments that set up the theme park's inscrutable firearm rules:
• In the premiere, the Man in Black shrugs off Teddy's hail of bullets.
• In Episode 2, Talulah Riley's robo-character tells William the pistols guests choose are "real enough, but you can't kill anyone you're not supposed to."
• In Episode 3, a bandit's shell sends William spinning to the ground. "I thought you said we couldn't get shot!" he says to Logan. "You can't get killed," Logan replies, adding that it "wouldn't be much of a game if they can't shoot back."
• In the Season 1 finale and Season 2 premiere, the harmless bullets begin to yield bloodshed. ("What about the fucking guns?" Charlotte asks Bernard in the Season 2 premiere, referring to the surprise killing spree. "Ford must have altered the system," Bernard says, "coded it to read all of us as hosts.")
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.