Last year we had a new to BP shooter turn up with a very nice .45cal TC that wouldn't shoot a 'group' smaller than about a foot a 50m with a REAL-style bullet.
It just so happened that I was bragging about my new, dirt-cheap borescope that fixes up to a cell-phone, and we used it. About 3/4 of the barrel from the breech up the rifling was completely filled with some kind of plastic stuff that had set hard, rendering the rifle into a mostly smoothbore. Much scratching of a number of heads ensued, but I advised him to use the old green-grade Scotch pad, and as much elbow grease as he could provide before trying to shoot it again. My advice was echoed by a number of other BP- shooters there, and dismayed at the amount of hard work ahead of him to clean this stuff out, he left.
The following Thursday - a BP-only shooting day - he was back, and asked me to take another look at the bore. TBH, it wasn't totally cleaned up, but I felt that another hour or so might well do it. What was the cr*p? I axed him.
The answer was astonishing, at least to me. He got in contact, via the gun store, with the previous owner, who admitted that he'd gotten rid of the rifle because it wouldn't shoot. Hardly surprising, when you consider that instead of using a regular patch, like most of us would, he'd used an undersized ball from his .36 revolver, swathed in many folds of Glad-Wrap.