@Null this is a response to last week's MATI gun talk. Slight power level, I have significant military weapons and small unit tactics training.
5.56mm NATO / .223 is more than sufficient for dealing with targets wearing modern L4 ceramic or steel plates. It will not be guaranteed to penetrate on every shot at any angle, but it is certainly capable of lethal penetration. The caveat is to keep your "fuck you" mags loaded with quality, high grain, high pressure ammunition. You can train with cheap ammo, and shooting more is better than shooting less, but at a minimum you need to make sure the man-stopping ammunition you use feeds through your weapon reliably and you have it in the weapon ready to go at a moment's notice.
Training is most important, more important than ammunition. If you're practiced and confident you can shoot your way out of a very bad situation with a .22LR. Never discount a threat based on what they're armed with, and when push comes to shove the gun you have is better than the one you don't. When you decide to shoot, you need to have the muscle memory that you don't stop shooting until the threat is gone. Especially for armored targets, you keep making follow-up shots until you're absolutely sure they're not a threat to anyone anymore. You should always aim center mass, but if you have the luxury of shot placement, basically no target you'll ever encounter will have protection for the femural artery, so aiming for the hip/groin is a good alternative to aiming for center mass. Shots to the hip/groin are likely to cause massive arterial bleeding, shatter the pelvis (causing extreme shock-inducing levels of pain), and damage/destroy organs. They are almost as lethal as a shot to the heart or head, but easier to land and almost never an armored target.
Modern combat experience has continually taught combatants that
economy of fire, not stopping power per shot, determines who wins and who dies in almost every engagement. In short, the more you shoot, the more you immobilize the enemy, the more you shoot, the more you win. Therefore, 5.56 is an excellent choice as it's one of the lightest rifle rounds while maintaining high penetration, and there are endless quantities of reliable 30+ round magazines available. That's the reason militaries around the world have switched to it or their own version (Russia uses 5.45mm, China uses 5.8mm). It's true that the Army is experimenting with the XM5 and XM250 that use an extremely high pressure round to penetrate armor at much longer range than 5.56 can, but in my personal opinion there's little chance they will fully switch over to that cartridge do to its higher weight and more limited magazine capacity. It's likely they will relegate it to designated marksman or special purposes.
I can likely answer any questions you have, but really I'd just watch Garand Thumb for more in-depth info. Other than being entertaining, he has a lot of real world expertise from a special operations warfighter perspective, makes good gear setup recommendations, and I agree with him on pretty much everything when it comes to tactics and training.
P.S. .50 BMG is an anti-materiel cartridge, if you want a "sniper" for defensive use, get a battle rifle in 7.62mm NATO / .308. .50 is best suited to firing into the engine blocks of cars. It might vaporize dudes but in the real world long range engagement you want the .308 so you can still kill the dude with the first shot but be ready to shoot again much much faster. Anything shorter than 1000ft you want the .223 so you can still be lethal and shoot even faster.