I've actually managed to shoot some ballistics gel tests of my own. Albeit, the funny thing is that, among two friends and myself -- all firearm owners -- none of us had an AR-15.
As mentioned before, I have a 30-30, which is the big bullet in the image compared to the smaller .223 which is the common AR-15 round. A friend of mine has an SKS that take 7.62, which is also a bigger bullet that .223.
We can announce with absolute certainty that several 30-30, 7.62, .357 magnum,.45ACP, and 9mm rounds managed to separate any chunk of the T-Rex ballistics gel mold from the rest of the body.
Obviously, everybody who knows anything about guns knows that velocity is a major factor in ballistics performance. .223 rounds are capable of greater velocities than 30-30 or 7.62.
Still, size is a factor, too.
This is the kind of lie that the anti-gun people tell that makes no sense. All it does is diminish their credibility. If my 30-30 won't rip the head off if a T-Rex formed out of ballistics gel after an absolute bombardment of dozens of rounds, it makes no sense that a .223 round would decapitate an adult human. Still, it doesn't need to. This is where the lie makes no sense.
Most bullet wounds inflicted upon a person's neck are death sentences. You don't need to exaggerate how gruesome such a wound would be.
Perhaps the gun control people feel the need to lie because they feel like it's a valid tactic in order to save lives. Well, it's not. The moment that you feel that you have to lie to defend your position, it's time to reevaluate your position.