I wasn't sure if this should go here since it's technically pistol related or rifles and carbines since it's also AR related...
ATF Rescinds Prior Methods to Measure a Firearm’s Overall Length when Equipped with a Stabilizing Brace
In its letter, ATF specifies that
[m]akers also create an artificial overall length measurement by attaching a folding stabilizing brace. Such a measurement would be problematic because the firearm could avoid classification as an “AOW,” yet retain the conceivability and remain fully functional. Measuring a folding (or telescoping) stabilizing brace would therefore undermine the comprehensive statutory and regulatory design of the GCA and NFA.…The measurement of a folding or collapsible stabilizing brace in the overall length of a firearm creates an artificial overall length that would permit a maker to avoid classification as an NFA “firearm” without a viable design purpose or legal justification.
It goes on to say that even stationary braces cannot be included in the overall length measurement, but the receiver extension can be.
Based on this letter, it is safe to say that ATF is taking the position that firearms equipped with stabilizing braces need to have their overall length measured with the brace folded or to the end of the receiver extension if the brace is stationary and non-adjustable. Adding a vertical foregrip to a firearm that has an overall length of less than 26 inches results in the making of an AOW, which is subject to the National Firearms Act.