5.56, typical rounds for ARs, aren't high caliber. In many states it's illegal to hunt deer with that caliber because it's under powered. Who told you it's high powered?
People absolutely use "high capacity" mags when these laws say more than 10 is too high. Take something like a Glock handgun typical capacity there is 15 to 17. For a modern rifle capacities of 20 to 40 is extremely common. For ARs the standard capacity is 30. Certainly any rifle anyone would consider for self defense is going to have mags larger than 10.
Overpenetration is a concern for any round a person would use for self defense. That includes 9mm and buckshot. Using 5.56 or 300 blk isn't actually that much more dangerous when just about anything is going through a bunch of wall if someone misses. And 5.56 might actually stop sooner than a slower moving handgun round because ballistics do weird things.