Hey @Dwayne indeed I’ve looked very hard at this particular issue. I’ll dump a bunch of links at the bottom, but if you have evidence to the contrary I am 100% willing to listen to learn.
To be clear on my point, I am not saying 5.56/.223 won’t go through any interior walls. I am specifically saying that a typical 5.56mm/.223 round will go through less walls than will a handgun round of almost any caliber. It will slow down from the drag of drywall, insulation, etc faster than a handgun round will. The reason is physics, the smaller mass means it conserves less momentum as it passes through barriers. They will also start to tumble and keyhole pretty quickly leading to further loss of energy.
Obviously, ammo choices play a factor. Birdshot is at best marginally lethal unless its really close range, but it will barely go through one wall. Legit defensive buckshot will go through several walls, and slugs will go through a LOT of walls. Similarly, a “real” rifle round like .308, .30-06, etc will also keep going through a LOT of walls since they are as heavy as a pistol caliber and launched at similar speeds to 5.56/.223.
And while I have not found an official statement to the effect, there are many references on the internet (take with a pinch or two of salt) that this is one of the major reasons that professional door-kickers (like SWAT) switched from pistol caliber SMG like an MP5 to AR15/M4 platform.
Link dump is below…
“About .223 Penetration” by R.K. Taubert, former FBI
(referenced in link above, site seems to no longer work)
https://web.archive.org/web/20191015225841/http://www.olyarms.com/index.php?option=com_content&task=view&id=14&Itemid=26