^^THIS^^Buffer tube wil not drive right through the shoulder; this is a (to quote Cooper) a poodle shooter .223, not an African dangerous game rifle.
Depending on the buffer tube style, it could be perfectly legal as a pistol.
People really don’t have to have a brace to shoulder. I know a few folks that have the longer slick buffer tube that won’t fit any stock or brace and short barrel. And shoulder the tube. With the back and forth tennis match ATF does on it they stay out of the funny papers that way.
It works better for a king range session with a vest or a sling as Hans mentioned but you can shoot a couple mags and your shoulder will be fine as far as 223/300BLK bigger ones probably less