1903 barrels with excessive erosion can result in blown through jackets, leaving a ring of jacket metal stuck in the bore. Subsequent firing irons that ring of metal into the bore so its not always visible.
This sort of erosion was a serious problem with the .30 cartridge of 1903 or any .30-06 cartridge loaded with the early HiVel type double base powders with 30% or more nitroglycerine content.
They discontinued use of those powders for military ball, but some long range match shooters still prefered them despite having to discard a barrel after each season of competition.
At one time a stuck jacket removal tool was part of every cleaning kit, but when they switched to Pyro-Cellulose and Dupont MR or IMR powder they only issued a few of these tools per company.
If a blow jacket or a layer of thick jacket fouling prevented solvent from reaching bore steel very serious corrosion would develop under the fouling. When that metal fouling broke loose or dissolved a very rough spot would be left in the bore.