Direct Impingement Gas Operating System with Rotating Locking Bolt, Integral Ambi QD Sling Mounts, Aluminum KeyMod Rail, SBX Pistol Stabilizing Brace. SIG\’s SBX Stabilizing Brace will give improved accuracy and reduced recoil when shooting an AR-pistol with buffer tube. The unique design uses the forearm to provide a natural and comfortable rest for shooters of all sizes and physical ability.