SIG Sauer 1913 Interface Folding Knuckle Stock Adapter

The SIG Sauer 1913 Interface Folding Knuckle is the bridge between the 1913 ecosystem and the much larger AR carbine stock ecosystem. It mounts on a 1913 Picatinny rear interface and presents a standard buffer tube thread on the other side, accepting both mil-spec and commercial-spec carbine buffer tubes. Add any AR carbine stock (Magpul CTR, B5 Bravo, BCM Gunfighter Mod 0, B5 SOPMOD) or any buffer-tube pistol brace (SBA3, SBA4, Maxim CQB), and the host runs that stock with a left-side fold.

The product is branded for SIG MCX and MPX but the 1913 input rail makes it universal: it bolts to the Sig MPX/MCX/Rattler factory rail, the Springfield Kuna rear end plate, the Stribog with factory 1913 adapter, the Scorpion with the HBI 1913 adapter, and AR pistols with a 1913 endplate. The combined assembly (knuckle + tube + stock) ends up around the same total cost as a Magpul UCS once you factor in the buffer tube and the stock, and it gets you the cheek weld and adjustability of whatever AR carbine stock you already own or know you want.

The trade-off is total length and weight: knuckle plus tube plus stock is longer and heavier than a dedicated 1913 stock like the FSA Single Folder Stock or the Magpul UCS. Pick the SIG knuckle when you specifically want a known AR carbine stock on a 1913 host, when you already own the stock and just need the adapter, or when the cheek weld of a Magpul CTR / B5 Bravo is the deciding factor. Pick a dedicated 1913 stock when you want the shortest, lightest possible assembly.