Not a fan of the FMJ's. Got no need to shoot thru cars.
But in reality, there are 2 things to consider: Concealment and cover. Concealment gives little to no protection. But it CAN deliver maginal cover- whereas cover is usually good cover.
Having been in a position to really A: Need cover, and B: defend agaainst a person firing from cover/ concealment, I will say that you can always shoot thru a target with extra firepower or penetration. But if you do not have enough power and penetration?? You are in a situation where you will not want to be.
For a small bore rifle, I would rather have the extra 'stick' of a slightly heavier projectile and hardier construction. With say a 30-06?? You will go clean thru a 12" TREE with a Remington Core Lokt or Nosler partition. Less need for a FMJ because of the power delivered.
A FMJ 223 in a 62-75gr. won't get there, no way, no how.
But, in the case of say being faced with a fight and a person is using a car as cover?? a FMJ 223 has a good chance of delivering thru the multiple layers of sheetmetal. A hunting round WILL start to expand, which will change trajectories, weight, and penetration immediately. It might not put the rounds where they are desperately needed.
So, this being said- I'd rather go with a very solid construction bullet, 'cause the last time I was shot at, every bullet counted.
And, so you know- a 9mm pistol with good ammo will penetrate a car door, trunk, seat, windows, gas tank..... and yet you might not hit the perp. Even when you send 15 rounds at him. We found that bullets went every which direction, except for straight once they were hitting even thin barriers.