According to the latest letter from the BATFE, putting a Sig Brace on an AR Pistol with the intention of using it a stock is illegal.
Until further notice, pending a change in this ruling (and/or my understanding of it), neither I, nor I.C.E. Training Company in general, will be advising on the use of the Sig Brace as a stock on an AR Pistol (or any other “short barreled” firearm) by our students.
This will include a restriction on their inappropriate use in carbine classes.
This is unfortunate, as the uses of the Sig Brace in that manner provided an inexpensive and simple way for people to own, and train with, a compact & powerful personal defense tool.
This position is congruent with my original position on the matter, as described in this video, recorded about a year ago, for Personal Defense Network. That position was taken based on the ambiguity of the way not only Federal Law is written, but also the unclear position of the BATFE.