Compatible & Incompatible: Compatible with 4" barrel compact to 5" barrel full size, work with 4.5" is best which like Glock 17, the barrel is neither long nor short; Incompatible with sub-compact or micro-compact. The bottom of the holster is open to support the longer barrel sticking out. High Safety: Level II Retention. It will automatically lock on the trigger when you put the weapon in, and makes a "click" sound to let you know that it is securely locked. Pull the weapon out quickly by pressing the button with your index finger.