Had the same issue with my R1 synthetic stock after I left the rifle in a case for an extended period of time (my mistake). I removed the rubber cheek weld, made a paste with baking soda and water, then used a tooth brush to scrub the stock and foregrip. It took about 30 minutes of elbow grease to remove the sticky residue. The hardest area to remove the residue from was the checkered areas for improving your grip, but it all came off. Stock and foregrip feel like new.