This is a short stroke gas piston with adjustable gas block. Look up the hk416, this was created by sig with the engineer of the 416. I have a sig 516 gen2 upper on a spikes tactical lower with a Franklin armory bf3 binary trigger. This is a three position safe,semi,binary with a crisp break. Has a vortex spitfire 1x prism optic red/green. Has a SilencerCo muzzle device and some other good stuff. Send me a message if interested. Three30-397-48five3