没见到具体说法,网上只看到 “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.” 安全起见,到最远点都不超过比较好吧。
还有上一句:Jeff also explained that the length of pull (LOP) needs to be kept in mind when building a pistol. The LOP is measured in a straight line from the face of the trigger to the rear of the brace. The rear of the brace is taken at its most extended position (if adjustable).
https://www.usconcealedcarry.com/blog/keeping-your-ar-15-pistol-build-within-legal-limits/