Looking to build an AR pistol, though not sure of caliber yet. After another member pointed me to TNArmsco, their Glock lowers fit well into my "Glock Addiction"

Can someone shed some light on the issue of AR "shoulder braces" for me ? Is this just a legal way to build a short barrel AR (would-be) rifle without the paperwork hassle ? It would SEEM that way to me, but that just sounds way too simple. Am I over thinking this ? Would appreciate any info.
Thanks, All the best for 2020 !
Ben