sorry Bill and everyone else
i see that person asking this same question on another forum ,was talking about a 54 cal barrel . my mistake .
But again , Bill
The case may very well be that the plug fits to the inside of the barrel and does not have an outer mating shoulder . Thus you will not see a line ..
Can you post a photo of the breech end of the barrel . If so , most anyone here could tell you if the above is the case .
Also as I mentioned , if this is a rifle like a Traditions CVA or Pedersoli , with a drum type bolster or flint liner , your going to have to remove that bolster or liner first even though with the Pedersoli , you should see a line where the breech mates , the CVA or traditions , you wont .