In general are breech seals on most rifles just ordinary O-rings or are they something special ? I've just received a replacement breech seal for a Walther LG55 and to me it appears to be just an O-ring, the type you can buy in bulk, assorted sizes on that auction site or should they be made of a special rubber/composite material or something ? The one I took out of the gun was hard and flat so I'm wondering if they become like this with use but started out just plain O-rings. Thanks.