https://safegunshop.com/product/ruger-ar-556-pistol/
AR-style pistol with arm brace.
If this was the weapon used--a pistol appearing to be a long gun--then I raised concerns about them last year. The ATF'nE was issuing regulations, but backed off due to complicated nature. With current power as it is in DC, this shooting could mark the end of them. Let us deal with this from a PRACTICAL angle, not an impossible and self-defeating libertarian or purist angle.
Cats, Guns, and National Security: A word about the pistol braces matter. https://catsgunsandnationalsecurity.blogspot.com/2020/12/a-word-about-pistol-braces-matter.html
In all truth, it would be better to lose pistol braces and hicaps on full-power rifle calibers than hicaps on regular pistols.
Be honest and tactical.
TDF INDEX: Cats, Guns, and National Security: THE DAILY FUDD index. https://catsgunsandnationalsecurity.blogspot.com/2021/03/the-daily-fudd-index.html