Question: is an adjustable pistol brace legal?
Answer: The rear of the brace is taken at its most extended position (if adjustable). “The ATF has advised that a braced pistol with a length of pull in excess of 13.5 inches may constitute a re-design of the brace into a stock.” ... Firearms in excess of 26 inches OAL may have vertical grips installed and remain non-NFA.