Interesting, I’ve pushed 7.62 by 51 (308) API rounds through AR500 quite easily. Like absolutely buttered through at 25 yards. .308 API vs AR500 steel - YouTube
FWIW, …True M193, is better respected then M855.
When I say true M193, I mean not all 55gr FMJ bullets perform like the M193 bullet.
Some 55gr FMJ bullets have thicker jackets, and that messes up possible fragmentation.
That said, pretty much any 5.56 / .223 bullet, hitting a bone is gonna hurt someone.
I prefer something like 62gr GD / 75gr GD. The Gold Dot bullets are a great performer.
The 55gr GD tends to be a good round for a 10’5" ish pistol.
IMHO, barrier blind type bullets are a good step forward over any .224 FMJ bullet.
Just look at typical hunting bullets… hardly anyone uses FMJ bullets to kill a Deer.
In shtf use what you have, the heavier the better IMHO, I’m former Navy MAA, I see my brothers from another mother here(Marines), we trained differently. If they have on armor, marines tell you legs Navy teaches you head shot all is fair in love and war. You show no mercy unless the threat is neutralized. But than again, I also practice with a laser training system from Strong tools, I’m placing a 9 mm in the space of a dime from 25’ consistently from a S&W CSX (3.1" barrel, iron sights) practice makes perfect 
get a good red dot, I have the Sig Sauer Romeo&Juliet on my PSA AR15, I have the same results, inside a dime@25’ and so can you, practice practice, practice.
remember. aim small miss small.
Whatever will go down that barrel, does not matter
9mm HP leg shot > full battle rattle