I wanted a non-buffer tube folding stock on my AR-15 lower that is dedicated to my BRN-180 (16") upper. Went with the ACE (Doublestar) system, consists of 3 pieces:
- AR-15 receiver block, that screws into the back of the AR-15 lower, and sets it up to take the hinge. I tried the "compact" version, but I didn't like how it effected the lockup with set-screws expanding the plug against the receiver RE threads.
https://doublestarusa.com/arrb-ar15-sig-556-receiver-block-a501- the hinge, I went with the steel hinge that has the boss. The boss (vertical mating lug) helps keep the hinge aligned with the receiver block. Steel hinge is VERY robust, weighs more than the aluminum version. a push-button would be convenient, but not avail on the steel version with boss. The hinge can be flipped over for either left, or right folding. I went with right-folder, since the charging handle is already sticking out on that side.
https://doublestarusa.com/fsm-s-steel-folding-mechanism-with-boss-a500s-k- the tubular skeleton stock. I went with the 9.5" stock because at the time, it was on clearance sale (no QD socket) and $30 cheaper. They also have 8.5" and 7.5". I did take a measurement of my AR-15 collapsible stock at my preferred position (2nd stop in), and it corresponded to the 9.5" stock. The stock attaches to the hinge with 2 socket-head cap screws (allen-head), doesn't have a mating lug, but the 2 screws hold it in place pretty darn good. not sure why ACE doesn't make it with a lug (boss).
https://doublestarusa.com/akfx-l-redesigned-ak-skeleton-stock-9-5-qd-swivel-not-included-a319-nsETA: 9.5" stock corresponds to AR-15 M4 stock fully extended. gonna order the 8.5" version. will post pics when that gets in.
the Troy QD socket receiver-end-plate helps to fill in the unsightly gap between the back-end of the AR-15 lower, and the hinge face. The rear-facing QD socket is covered up by the hinge when the stock is swung open, so just the 2 side QD sockets are usable.
just thought I'd put this out there, for what it's worth, YMMV.