Stolen from Reddit:
According to the source of the discoverwestworld.com, the following is mentioned in the terms of Delo Destination:
(c) All weapons and equipment used within Delos parks are the exclusive property of Delos, Inc. Gun ammunition contains proprietary safeguards related to bullet velocity, and tampering with gun safety features or ammunition automatically transfers liability to you and absolves Delos, Inc. of any injury or death that may occur as a result.
Also, as said in the interview by Jonathan Nolan himself:
In the original film, the guns won't operate guest on guest, but we felt like the guests would want to have a more visceral experience here. So when they're shot it has sort of the impact. They're called simunitions. The U.S. military trains with rounds like the ones we're talking about. But there's a bit of an impact, a bit of a sting. So it's not entirely consequence-free for the guests.
So I guess the guns have safeguards in them that can recognize humans versus host and change the speed of the round depending on what it's being aimed at.