I've seen a few people in various wounded states thanks to half of my family either working or having worked in the same hospital near where I live, including my single mom. I've also had the good misfortune of seeing a few die. It's obviously not a good direct example because of the fact that the sympathetic nervous system's usually already shut down, but I have heard plenty of 'this just happened' stories about people getting all kinds of varying injuries.
Most people get fucked up pretty badly pretty quickly. A single round of pistol ammo, even 'light' pistol ammo like .320 ACP, can usually put someone down if they're not in a fight-or-flight state when they're shot. Of course, most people in a fight for their life are very much in fight-or-flight mode, and it can sometimes take several .45 ACP rounds or up to take them down. Anyone on a good amount of something like cocaine or meth is basically invincible to anything that doesn't outright cripple them like spine or brain damage.
Soldiers are a bit weird, though. The closest thing I can compare them to is gang members. The problem with these guys is that they can basically run out of fight-or-flight if they're in an engagement long enough. The body can only store so much adrenaline, epinephrine and norepinephrine and once that shit runs out it takes a while for them to build back up. After an extended firefight, gang members literally get easier to take out (assuming anyone can even shoot straight enough any more, gang members aren't exactly known for their training), and I imagine that the same can be said for a soldier.
As for STALKER, you're playing on the highest difficulty setting, right? The rest of the difficulty settings are... 'broken', to put it nicely.