S size. Positive you may need some force, but this is due to The truth that the quick adjust buckle is held set up by friction. This might be the weak point to someone but I have found this to get of no issues for me, the sling hasn't missed https://blue-force-gear-vickers08518.mybjjblog.com/top-latest-five-holosun-509t-reflex-sight-urban-news-47305964