Glad you asked! There was a time when I and a bunch of gun nerds had way too much time on our hands (before Youtube, sadly, or we would probably have a pretty good channel), and built a bunch of different test specimens and tested a whole bunch of different rounds and calibers.
.223 DOESN'T penetrate as well as you would expect in most cases, because it depends heavily on the angle of incidence of the round, and the design of the projectile. A ball round fired straight at a light framed wall will penetrate several layers of gypsum board with enough energy to cause injury. It DOES shed mass and velocity with each layer, but you should expect deadly effects if there's only one wall in the way.
At an angle it changes dramatically. If the projectile strikes a wood stud solidly, a ball round will fragment, and deposit most of its energy in the wood. On the test specimens I've used, it will deform the back layer of the wall, but won't penetrate with any apprciable energy. Same goes for solid blocking which is typically installed in wood framing.
The story changes with different projectile designs though. One would want to build a test specimen and try out different ammo for himself, but there are brands of frangible ammo that I've shot that really do dump most of their energy in the first couple layers of drywall, but it's expensive. Federal used to load a heavyier (IIRC 77 grain) hollow point that when tested just broke up into a bunch of pieces that did penetrate the second layer, but lost a lot of mass.
So, I would say that Sarcazmo is mostly correct, based on my own experience. But, mainly, don't miss. Put the rounds center mass in the bad guy, and this shouldn't be an issue.
See post #38 for link.
I'm referring to defensive expectations, not cautions on offense.
Wood and sheet rock is concealment NOT cover.
Thanks! You saved me a lot of work.
If you can call shooting things work! XD XD