This was discussed with showrunner Jonathan Nolan in an interview with Rotten Tomatoes. In short it's the bullets that are special, not the guns.
With the same guns, the guests can blow the hosts away, but then the hosts still come back the next day.
“It’s not the guns,” Nolan said. “It’s the bullets. We thought a lot
about this. 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.”
11 RULES OF WESTWORLD — HBO’S KILLER ROBOT SERIES
The waiver on the HBO Official "Westworld" website offers the following info.
2. (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.
And there's also this snippet on the DelosIncorporated viral website relating to "bullet velocity protocols"
AGGRESSION BETWEEN GUEST ID#398436 AND GUEST ID#435873 HAS
INTENSIFIED. GUESTS NOW ACTIVELY SHOOTING AT EACH OTHER.
BULLET VELOCITY PROTOCOLS FULLY OPERATIONAL, AND GUESTS REMAINS
UNAFFECTED, BUT MUST DEESCALATE SITUATION.
So there you go. Clear as mud.