In the body of the comment he wrote
I recently bought a modern sporting rifle...I asked a relative with more knowledge if I got it what I should put through it. Without hesitation the answer was the 62gr steel core over the 55gr.
Time has passed and I have viewed lots of videos including ones from Paul Harell on youtube. I watched the effects on the "meat target" and now am confused. I realize that I failed to ask important questions. Target, overpenatration, effectiveness, range...
While I do have a few opinions, I do not qualify as any kind of expert.
My advice: The "best" ammo for a modern sporting rifle is any ammo that is available in quantity and is inexpensive enough that you can-and-will practice on a regular basis with that ammo.
Unless your target is shooting at you, you cannot justify shooting "targets" at 100 yards. In a court-of-law you would have a difficult time justifying shooting targets at 20 yards. The only reason range would be a factor is that projectiles lose velocity as they fly and the terminal ballistics change. So, let's call "range" a tie-breaker.
I think this is far more of a consideration for police than for civilians for the following reasons.
A policeman does not know the layout of your house or where other, nearby houses might be lurking. You do. You know where your kids' rooms are, where their beds are relative to your own. You know where your neighbors' houses are.
You cannot dictate where your target is but you can certainly change your own position. And don't overlook changing vertical position. If you are in a house and are compelled to shoot through a door, then plunking your ass on the floor and firing upward will send misses and pass-throughs into the ceiling. That is still an issue if you are in a multi-story dwelling but it gives you one more set of options to consider.
Am I advocating shooting through a door? NO! But maybe you have special circumstances. Maybe you have wireless video cameras and you are 99.999% certain that the "target" on the other side of the door means to kill you or render grievous bodily harm to you or your loved ones.
The other consideration on over-penetration is that with handguns, anywhere between 75% and 95% of the bullets fired at a "target" miss that target and fly downrange with their full velocity. The best way to address issues of over-penetration is to ensure center-of-mass hits of the intended target. The energy transferred to the target is not available to cause collateral damage.
Terminal ballistics will be argued about for as long as there are people who like to argue.
It would be silly to argue that all projectiles perform the same. One consideration that is often overlooked is that targets vary a great deal. Picture a four-hundred pound target throwing a chunk of concrete in your direction. He is right handed. Where is his left arm as he is winding up to throw? His arm is in front of him and there is a 10% chance that your shot will hit it.
Now consider a 110 pound target with a switchblade. The projectile that is optimal for the 400 pound target is sub-optimal for the 110 pound target.
The traditional advice is still the best advice. In a crowd of targets, identify the biggest threat and service it with as many shots as necessary until it is no longer a threat. Then service the next threat.
The military figured this out a long time ago. Servicing threats requires focus. Identifying which threat is the most imminent requires a general over-view. Snipers have spotters. A team of two is eight times better than a single, armed person. One person is the target caller and the other is the trigger-puller.
Unrealistic? The only time my demographic goes to the shopping mall (likely Scenario #1) is when dragged there by the most important person in my life. She doesn't want to pull triggers but she certainly doesn't want to watch my noggin get crushed by a chunk of concrete. She will gladly call targets.
The steel tip on the M-855 projectile functions like the cork or paper-packing in the tip of the old .303 British bullets. It moves the center-of-mass further aft and increases the odds and severity of tumbling. Tumbling is triggered by the tip hitting the target at angles that are not perpendicular which exerts a side-force on the tip. Sometimes the bullet hits the target square-on and it does not tumble.
The FMJ 55 grain projectile fractures at the cannelure as it tumbles. It will not fracture if it does not tumble and fracture is contingent on having enough velocity when it hits. It might not have enough velocity when fired from an AR based pistol.
55 grain softpoints have a maximum penetration of 10" to 12" which is just shy of the FBI minimum. Still plenty if you are prick-punching the target between the nipples and are not shooting them in the beer-gut or engaging in "raking shots".
Any .224" projectile marketed as a "varmint" bullet should be avoided for this application...unless that is the only ammo you can find.
Practice, practice and practice some more
Maybe you can afford to practice with "the best" ammo as much as you should if you work for an agency who happily pay for it.
Most of us cannot afford to shoot hundreds of rounds a month of the "best" monometal or bonded ammo.
Practice will verify that your ammo-of-choice functions flawlessly through YOUR firearm.
Practice will greatly increase the odds of YOUR firearm becoming an extension of your body that does not require conscious thought to operate (muscle memory).
Practice will greatly increase the odds of your eye, the sights and the target's upper center-of-mass coming into alignment as the weapon settles into your shoulder pocket.
Practice, practice and practice some more. The ammo failure won't be inadequate terminal ballistics. The ammo failure to fear is having all of your ammo in the magazine as the crowd of targets rips your weapon from your hands and beats you to death with it because you could not bring it into play quickly enough.
The ammo failure to fear is jamming.
The ammo failure to fear is missing the target.
The best ammo for a modern sporting rifle is the ammo that you can find in sufficient quantities at a price that allows you to practice as much as you need to.
It will probably be either 55 grains or 62 grains. It may be military surplus or production over-runs. It may be the "economy" brand from a major ammo manufacturer.