well IMO the highest pressure and hitest temps in burn ar at the breech to within about 12 inches of the breech .
the further down the barrel the charge moved s , the lower the pressure and the lower the temps . this is speaking of BP of course . synthetics act a little differently .
Inspecting original iron barrels its not uncommon to find this original area where the ignition starts , to be larger . This can be from poor cleaning practices or just plain use .
With modern steals , something many folks don’t think about is loading and RR use..
Some times if we think about it we find that the wear comes from being consistent . IE
A) we use X length short starter that pushes the ball to that spot
B) this is the area where maybe your RR drags when removing it
C) when cleaning sometimes , I know with myself , ill find myself swabbing the bore and then realize I was only scrubbing one area more then others
D) Lube . Ya lube , namely things like bore butter. See when the lube gets old , it gets hard , it collects dirt and dust . Then when you load it , It get slammed with the short starter to a point in the barrel . Then taped with the RR . This I believe causes a slick area to form as this is may be the area that the dirty lube is pushed away
Now if you have ever shot conicals ? Leading can also act in strange ways and will effect accuracy
The other thing to consider is was the rifle ever accidentally short started. this can happen and happens more then we want to think .
If this happens 1 of 3 things will happen
1) Nothing to the visible eye . But if one uses one of those longer short starters , over time the pressure can effect the barrel for a few inches just back of the area where the ball stopped .
2) the barrel acorns . .
3) the barrel fails completely and its walls get breeched
I would bet though you probably will find the cause though in A, B, or C rather then 123
There are many things that cause things to happen , your problem could be any number of them
But IMO to often we say . ‘I shot the barrel out “ when really what needs done is the barrel freshened .
IE Lapped , leading removed . Plastic removed .
Yep plastic . If your not using 100% non synthetic blended patches or wads , your burning plastic, just as any sabot shooter would be doing .
Again , a lot of things could cause this . But my point is , very few folks actually shoot enough to have the shooting alone , wear the bore