The ST45 is an ambidextrous 45 degree safety selector for the AR10/ AR15 platform. Instead of having a full 90 degree rotation to disengage the safety, this selector is set to fire at around the 45 degree point. This allows the operator to flick the lever to disengage the safety quicker. This safety selector is incredibly smooth and has a super crisp lockup. It is designed for firearms that have the correct FIRE markings on the receiver to match the 45 degree fire position of this safety selector. Do not use on lowers without the 45 degree ,FIRE, markings on the receiver. If you have a receiver that does not have the correct markings, you should use the FT90 safety selector.