The rubber end piece on the 550 buttstock is subject to damage or crumbling because of old age. The rubber buttstock end piece can be replaced. Put the butt pad in a vice, as per the picture. Be sure not to touch the plastic part. Clamp tightly. Give the stock a quick slap in the direction of the arrow and it will detach. Then, place the new end piece in the lower part of the stock and gently tap with a rubber mallet. This info came from a Sig 550/551 Manual.