Yankee Hill Machine Co Sidewinder Direct Thread Adapter, M18X1, Converts the Sidewinder, Cobra M2, and Cobra to Direct Thread, Black YHM-3252 Simply Replace your spring retainer" spring and piston for this fixed thread mount. These adapters make it possible to use any caliber Sidewinder on Fixed barrel pistol caliber rifles and pistols. Made from 17-PH stainless steel with a Matte Black Finish.