I just watched a Modern Marvels about Bullets on History Channel and they discussed hollow point ballistics in the human body. I don't recall that they said over-penetration was completely resolved but they did discuss that HP rounds were effective due to larger cavity (resulting in more massive internal damage and faster bleed-out - thus incapacitation of the target) and that the round was less likely to injure bystanders due to over-penetration. You might look at their website for some documentation.
The only ones who can make that claim are the makers of frangible ammo, which disintegrate on impact, but they are less than desirable for self defense. As for other ammo, it depends mostly on shot placement. Put any other bullet through a body without striking bone and chances are it will keep on going.
HP's are certainly more likely to stay put, but you can not count on it. Put some HP's through leather, denim, dry-wall, etc, on the way to the meat and the HP can clog and act like a FMJ. I've seen it happen many times.
Every human has a different density as based on musculature, fat and water levels. Ask any doctor or surgeon.
Further we all have different body cavity sizings and density too. Ask any tailor.
Add to that multiple _different_ point of impact possibilities as people are not always shot straight on but at angles too. Ask a coroner or person who has worked combat or ER triage medicine.
The totality of all these can and does add up to so called 'over penetration', very easily.
This applies to all living beings as shot, not just humans. Ask any hunter.
It is real science. Can happen. Does happen.
To expect or doubt that as much could happen is to ignore basic physiology and physics, regardless of bullet design and manufacture method.
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