I am selling 30 S-size Tai Rubber (Tairaba) caps. These are equivalent to the Jackall Bin-Bin Backfire Cap S size. They feel slightly larger than Jackall's. To make them M size, you need to drill them with a 2.9mm drill. (Please do the modification at your own risk.) The size is based on my personal experience and subjective opinion. When considering a purchase, please check the size in image 3 before buying. No price reduction. There may be manufacturing burrs, so please remove them before use.