sounds like the area around the nipple has suffered from scorch and burn from the cap going off and burning the bore butter.i went through this years ago because i only used bore butter as per the thompson center instruction book on the outside and in the bore. if you use a good gun oil on the outside of the barrel this wont happen. as far as long term storage go with gun oil.just make sure you get the oil out with 90% rubbing alcohol before shooting again. i only use bore butter as a coating in the bore during the couple week hunting season . the rest of the year it gets oiled.