I think, even without knowing the steel alloy, that barrels today are likely to have around .4% carbon. It would seem that's enough carbon to change the structure of the steel to something that was unintended if it was locally heated like you describe. Most all barrels have a very low rockwell hardness number that most folks would consider annealed. Some alloys will also air harden to some extent, possibly locally. It's only thoughts is all.