An improved Rail Adaptor System/Rail Accessory System (RAS) which attaches to a firearm. The rail is top opening, modular, and free floats the barrel. Provided is a rigid, lightweight, strong platform for mounting firearm accessories. Heat transmission from the barrel assembly to the user is limited. The user is also protected from ventilated gases originating from the operating system. A quick detachable top rail section is provided so that the gas system may be easily accessed. This removable top section of the rail is what makes this device unique because the RAS may be installed without removing the barrel, gas system, front sight base, flash hider or the barrel nut. The herein described RAS is adaptable to a wide variety of firearms with the use of conversion parts. The top tail “returns to zero” on reinstallation allowing the remounting of various optics and electronic gun sites without the need to realign them.