The general statement about the mythical wounding potential of the typical round the AR-15 fires, specifically: "The specific example was of even if someone got him in the forearm the entire arm up to the shoulder would be useless because the damage would travel upward or something" is a load of horseshit but the media probably doesn't know better. Most any bullet can make an arm useless it just depends what it hits in your arm.
There is a grain of truth within that shit sundae however.
Start here.
The mythical claims of damage from wounds referred to here is probably the fact that the small projectile that .223/5.56 typically expends downrange will
tend fragment during impact with flesh if impact occurs above a certain velocity (depending on specific .223/5.56 variant, not all bullets are equal by design) compared to some other common rounds designed for similar purpose weapons (
7.62x39 for example, commonly used by the AK-47). Given that the name of the game with bullets is to get them to expend all their ballistic energy into the target (hunting rounds are typically "soft point" and designed to start expanding on impact for exactly this reason) this is certainly one way to go about doing so. Note the "above a certain velocity" caveat though. Bullets lose velocity as they travel so out past a certain range it's going to lose that fragmentation effect.
Of course as that's just how the cartridge tends to behave fired out of any gun chambered for it, no magical wounding powers imparted by it being shot out of an AR-15 or M16 platform gun. And the cartridge is far from unique to the M16 or AR-15 anyway, as the high velocity generally low weight projectile makes it a fairly flat shooting low recoil cartridge ideal for
"varmint guns", though its genesis was ultimately a result of research that led to the M16. It's that association with the M16 where the "DESIGNED TO DESTROY HUMAN FLESH" thing comes from.
For context, the M16 was a result of the general consensus that battle rifles of the time (FAL and M14 being good examples) were overpowered for the ranges that combat actually took place at (typically sub 600 yards, often less than 300) and thus the need for the heavy ammo and comparatively harsh recoil from those battle rifles was actually a problem in need of fixing. The keyword here is "overpowered". If you compare the size of
7.62x51 to the 5.56x45 (7.62x51 used in the FAL and M14), or
5.56 vs. a 30-06 (cartridge used in the WW2 battle rifle, the M1 Garand) you can begin to see why.
Some proposed solutions involved
flechettes (even smaller diameter, faster flying, metal darts) but were ultimately dropped. Eventually they settled on the M16's general design and cartridge given the weight savings, decreased recoil (and thus one would assume, an easier to control weapon), and what were considered sufficient results during ballistic testing (see: high velocity wounding characteristics), keeping in mind that the military isn't hunting elk or moose, they are hunting people, and only need ammunition powerful enough to do that. So a lower powered cartridge is "good enough".
Hence, the "DESIGNED TO DESTROY HUMAN FLESH" meme. It's a less powerful round the military selected BECAUSE IT WAS SEEN AS GOOD ENOUGH and the benefits (logistical, weapon control, etc.) outweighed the negatives,
not necessarily because it was "better at destroying human flesh" (that's the M855, the round the military went with instead of the M193 from the video linked earlier) than it's
larger competitors at the time. Note that in those videos the "(number)gr" is the weight of the projectile, keep that in mind when thinking about logistics.
It is worth noting that there continues to be debate about whether the 5.56 is actually a "good" cartridge for military applications as there have been persistent complaints about the stopping power of the cartridge, especially out of shorter barreled weapons (remember that "above a certain velocity" caveat? shorter barrel, less velocity), all the way from 'Nam to present day, recently leading to private development of alternative cartridges such as the 6.8 SPC to address this perceived need during the second Iraq war. I don't want to get into that aspect of it too much as there can be a lot of bullshit bandied around where people start talking about "stopping power" but some of the arguments and responses to them can be found in the "criticism" section of that wikipedia article.
Related to mythical wounding characteristics out of tiny cartridges,
look up the 5.45x39 Russia's answer to the 5.56, if you're so inclined.
