As noted by Bart today, it has come to light that the
ATF has gone after an 'illegal SBR' for having an angled forward grip (Magpul angled grip) and an 'adjustable cheek rest' (Maxim CQB PDW Pistol Brace).
The man was acquitted, but still had to pay to fight it.
Most notably, the ATF wanted all of their determination letters about angled grips and pistol braces to be excluded from the case, in order to 'avoid confusion'.
Not everything that the ATF does is going to be featured on YouTube...