SIG MCX/MPX Folding Magpul SL-K Stock and Adapter

The SIG MCX/MPX Folding Magpul SL-K Stock with Adapter is the factory SBR conversion stock for SIG MPX K and Copperhead owners converting their pistol-braced gun to a short-barreled rifle. The SL-K stock itself is the same Magpul SL-K used on AR-15 builds, mated to a SIG-built 1913-interface folding knuckle that locks the stock open under recoil and folds to the right side of the receiver for storage and transport.

Three benefits matter on an MPX. First, an actual stock with a fixed cheek weld is dramatically more usable for sustained shooting and accurate fire than the SB Tactical pistol brace it replaces. The SL-K is short, lightweight, and has a positive cheek-weld surface that keeps a red dot index consistent. Second, the locking hinge eliminates the wobble that plagued the original 1913 folding stock variants, which was the primary complaint about the early Sig folding hardware. Third, the stock telescopes to six positions, so a shooter wearing armor or a heavy jacket can extend the LOP for a proper shoulder index without resetting the gun.

This is the path most MPX K and Copperhead owners take when they go SBR. SIG sells the same hinge as a standalone 1913 adapter for shooters who want to pair a different aftermarket stock to the MPX, but the bundled SL-K kit is the price-competitive package most owners buy.

Compatible only with the MCX and MPX (and any host using the proprietary SIG 1913 interface). Installing it in place of a pistol brace makes the gun an SBR; federally that no longer needs ATF approval, but check state law first.