This is an aftermarket kit designed to fit all of the parts of the marker including the bolt and any regulators. All of the O-rings in this kit are made from Buna-N rubber and are in the same durometer / hardness found in the markers manual or from the original marker itself. Most of our kits now include extra o-rings based on common aftermarket additions, customer requests, and or o-rings that we feel are more heavily used in the kit. Our kits do not include any Square rings, X rings, Quad rings or U-Cup seals unless specifically stated. The insert includes a list on the reverse side explaining placement within the marker. Why buy o-rings at all? At the beginning, your o-rings have a nice tight seal and all of the air pressure is directed to the paintball as it should be, causing your paintball to be straight, fast and precise. Though overtime the seal is not as good and air can leak out around the now imperfect seal and that causes your paintball not to fly as far, as straight, or as accurate as it did when these seals where new. You might not even notice the difference because you have seen it change slowly over time so you have adjusted to how it is working each time you use it, but in reality you might have lost 10-30 feet per second of speed and 10-30 feet of ball travel. Losing any part of your shot is no good at all! This is especially important if you are in any kind of competition with your marker. So it is important to change o-rings out often. **Note: The picture is an example kit and example contents for an o-ring kit. Your kit will be for the marker/regulator named in the title. Sizing chart shown in picture can be printed for free directly from our website.