probably put in by a gunsmith. i know you can buy screw in ones just like a regular choke tube, my friend has one on his gun and really likes it. if you wanna use a full choke you can just twist it and there ya go, if u wanna go back to a modified or somethin just twist it right back. u dont have to worry about carrying around multiple choke tubes. i thought about buyin one but it was like 90 bucks or somethin like that.