Almost completely relevant column
I know a guy who works at the zoo and while its usually kept a secret, they have weapons and training in just how to take down any animal whether it be a 400 pound gorilla or a 2 ton elephant.
I served in the Canadian Forces with a guy who was forced to shot a 600-pound polar bear with a C7 (an M-16, basically.) He killed it with one shot. He got a bit lucky, but it can be done.
I am quite confident it could, but getting a successful kill shot on a big crocodile in the dark would be rather hard even if you had a .50 cal. Anyway, the extent of the crocodile fatalities there has been insanely exaggerrated.
We have a feral hog problem in this part of Missouri, and a relative of my wife’s has had his land torn to shreds by those beasts. He’s killed a half-dozen or so, using semi-legal military-grade weaponry. He says that anything less just pisses them off more.
This reminds me of an incident which occurred in the late 1980’s/early 1990’s where my young cousin was badly injured by a rampaging bull at a local market, my aunt said that the army who patrolling the area at the time had a lot of trouble stopping it, as she described it the weapons they had at hand weren’t effective (or at least not quickly enough) and “they kept bringing in bigger and bigger guns”.
At the very least it taught me respect for our bovine friends.
Forgot to add, this was in Northern Ireland during The Troubles so that’s why the army were at hand, I think its one of the few times the locals were glad to see them.
One properly placed round from a .223 will kill a bear or gator. Now, sure, in order to* stop it, kill it on the spot, *a burst or two may be needed.
So, the film is fairly accurate in that.
I remember one poster who had gone to North Pole (maybe robby?) on a submarine daying that they had a Polar bear watch and he expected the M16 wielding sailor to have no real chance against an actual Polar Bear if it showed up, though I cannot recall if doubts were due to the bullet calibre or the sailors perceived lack of marksmanship.
Depends on the viciousness of the animal as well. Remember a couple years ago when some guy that had an animal rescue in the Midwest released all his lions, tigers, and bears into the open before he committed suicide? The cops had to go in and kill most of them. I assume they had maybe 9mm, maybe something a little stronger, but anyway one of the cops made a statement about how basically the hardest part was taking down the tigers. While the other animals would back down or run away he said the tigers were so aggressive they would keep coming until they were dead and they had to unload entire clips into them.
Tracer rounds can set sheep on fire, so there’s that. I don’t know if it’s directly related to the discussion at hand, but the possibility of facing a wounded, pissed-off bear that’s also ON FIRE exists.
Cops have a lot more than 9mm these days. In my (not violent, very safe) SoCal city the motorcycle cops have M16s (or maybe M4s) in an external scabbard. I assume the cars have them in the trunk.
Reassuring in case the LA Zoo gates burst wide open.
I’m putting this in my “Really Useful To Know” file. 
I’m not calling bullshit. I’m just seriously asking for more information here, because i didn’t think sheep were that flammable.
Well, perhaps, but I’ve seen many a beef critter dropped instantly with a single .22LR (hogs too). Granted, this was for slaughter, and the animals were standing still enough for a shot between the eyes at point blank range, but that just goes to the point about anatomy vs bigger guns.
A guy I met is a door gunner in the Air Force, flying missions behind the lines to extract SF teams in Afghanistan. He said that SOP on these missions is to test their weapons partway in, when they’re over uninhabited areas, to make sure they’re working. On a night op, they tested their guns (which fire a mix of rounds, including tracer) and he saw this weird, amorphous heat signature on his scope. He had inadvertently fired into a herd of sheep…
Yeah, well I’m calling BS.
As soon as the tracer burns out the flame will extinguish.
You can easily test that yourself with some wool and a match.
Flames will not move from a sheep’s fleece to another.
More likely he’s fired into some dry vegetation or hay in a pen where the sheep are constrained and set that on fire.
He was probably just seeing the sheep’s body heat.
While I imagine that full auto would reduce the concerns, it’s illegal to hunt deer with 5.56 some places.
Ockham’s Razor in action! 
Moderator Note
Let’s refrain from attacking other posters for no good reason. Given that the question was prompted by a specific scene in a movie, there is reason to mention the particulars.
Colibri
General Questions Moderator
A comparison of muzzle energies with two rounds that were popular among deer hunters I knew when I lived in Michigan:
12 gauge slug - ~4200 Joules (potentially quite a bit more for the longer magnum rounds)
.30-06 Springfield (M1 ball as used in the M1 Garand) - 3,627 Joules
5.56mm (M855A1) - 1,889 Joules
Terminal ballistics matter. That’s how the bullet moves once it hits. There’s a host of important little details that can affect lethality. Just as an example, overpenetrating in the movie means the bullet carries a fair amount of energy out the backside of the hippo, gator, were-bear instead of dumping it internally into flesh. Potential terminal ballistics differences aside, the 5.56mm NATO round is on the weak side compared to typical hunting rounds used for large game. In many states it’s illegal for hunting large game. It’s less powerful than used to be the norm in military rifles. It just wasn’t optimized for killing large game or humans with a single hit. Power and lethality got balanced against other considerations.
Since it sounds like they were mostly dealing with [del] hungry, hungry[/del] blood crazed creatures attacking at close range that’s still a pretty effective design. It’s not a deer hunt where you sit and wait for that one good shot. Up close M16M4s can fire more often on auto while still having a good chance of hitting with any given shot. That’s a lot more holes with enough power to get deep enough to reach and damage important internal organs. That helps make up for each individual hit potentially doing less damage. Each hit also has a chance of directly hitting something that’s critical (brain, heart, major artery, etc.) enough it will close the human buffet relatively quickly.