After watching all 10 episodes, there appears to be either several weapon "safety" categories in Westworld, or there are some small inconsistencies. I've spoiler blocked some stuff, mostly relating to s1ep10, so if you haven't seen the whole first season you will have to read around it.
Most westworld guns shoot a low velocity non-lethal bullet at humans but either a high velocity lethal bullet at hosts or the host has implanted squibs to give a blood splatter effect followed by a software "I'm dead" effect to the same low velocity round. RMalke's answer has showrunner quotes which suggest the former. So the gun has an ability to identify friend from foe (IFF, though in this case maybe it is IHH, Identify host from human :) that tells it how fast to shoot a bullet. This has to be independent of the user, since a gun can be wielded by a host or a human, and aimed at a host or a human.
The security team carries modern design pistols with red barrels. These presumably shoot regular bullets lethal to humans and hosts alike. Ep 10 emphasizes this with the P90 submachine guns that the hosts use against humans to astounding effect. It is possible that these hosts were already able to override the weapon safety but then why would there be red barreled firearms at all? I can only conclude that the red tip indicates a normal functioning firearm, one without any type of IFF.
BUT Delores has a "western" revolver at the end of ep 10 that she uses to
shoot Ford in the back of the head, as well as several guests at the gala, all of whom are presumably human. She also shot Arnold with the same pistol decades earlier. You might conclude that this particular pistol has no IFF safety but Old William is also shot by a host wielded weapon at the gala, probably an IFF equipped human safe weapon, not some red tipped gun taken from a security guard (though we don't actually know).
So either the host can override how the western weapons work or somehow even the human safe guns were remotely re-programmed in order to be able to shoot humans (which seems to be very irresponsible on behalf of the Westworld team). I don't recall an actual human on human shooting in Westworld to prove that a human can't shoot another human with a westworld gun, maybe
young William against Logan?
but it seems unlikely that this is possible.
So apparently either the host can override a firearm's internal IFF safety or there is a slight show inconsistency about how the IFF works since a human would be able to hurt another human if there is no host to control what the gun does. A host shooting a human to no effect isn't just about aiming to miss or that they have special weapons that are different from what humans use since there is plenty of host on host lethal violence as well.
So I think that the guns IFF is mostly a handwaved concept that is subject to a hosts intent. It is certainly
Old William's desire to have hosts be able to hurt humans, probably Ford's desire as well,
so it is no surprise that it happens, but the actual mechanics of how the IFF is disabled (central computer hacking or local host override) isn't clear and may not be strictly internally consistent.