If they’re inside your home, all questions & warnings are complete. They’ve deliberately, with consideration & effort, breached your social & physical & legal boundaries. There is no need to “warn” them (they know the risks), there is no reason to keep a chamber empty just so you can waste time “warning” someone by filling it (when you may already need that round chambered), they have likely already prepared to confront you so you’re “warning” them of what they’re ready for.
.223 / 5.56 is notoriously unstable, likely to start tumbling & lose energy fast when having struck the first object - it’s not going to “pierce the walls of a dozen trailers”. Overpenetration has been well-addressed for this caliber, and is proven better than most.
A major point of using a carbine is accuracy & maneuverability: increasing odds of hitting the intended target, and so reducing other casualties.
Shotguns, however, have: limited capacity (don’t miss!), deep penetration (for some ammo), and the very same (& debatable) “spread” people tout as improving hit odds also improve odds others will be hit.
I should not have said rack a round. I keep one in the chamber and I’m quick enough that you can’t hear it rack. If you don’t hit any studs or furniture a 5.56 could go through a few trailers. Not much to a trailer but after Hurricane Andrew ripped up Homestead they did start making them better.