Oh, and the Sig M400 is a fine weapon. Pair it with a Sig Romeo 5 red dot.
I really like my Sig M400. Seems to me it hits harder than the M-16 I was issued in the USMC back in the 80’s, not sure why, must be the modern loads. But yes, it will definitely go through the perp and also the house or apartment walls, unless they are made of concrete. A 7.62 x 39 (minimum 18.5” barrel length) is good for reaching out a little bit farther than an AR, hence, inside, that would probably go through the perp (if it does not contact bone and tumble inside) and the walls as well. A nice large caliber handgun (40 or 45) with hollow points is my “go to” for home defense. Wife has a 9mm (with hollow points) as well.
Sig Romeo 5 red dot is very reasonable...$119 at Palmetto State. (Pre-tax and shipping probably)