I was at the doctor and of course had to wait.. ( that is why they call us patients because it takes a lot of patience !!) As I was waiting I scanned the room and looked at the ear cones for examination and notices the size of one, 4.5 mm.. When we were finished I asked about them and he said help yourself.. Found one that fit in the breech of the barrel.. Cut it back so my cleaning rod would go though.. Pushed the cone into the chamber and it fit perfect.. Cheap guide for a .177!!!