Paul - I hate to say this but I really doubt the problem was the T7. Build up or not - if the bullet loaded easily (got past the build up), then shooting it would not have caused a problem. Personally, having worked with all types of forgings and metal working, would have to say that that crack was in there when you got it and was missed during inspection. Or, it may have been a hair line crack that could have beeneasily missed or was just subsurface and your shooting it caused it to open up.
In any event, glad there was no injury to you or anyone else and that you got a new barrel and it is shooting good for you.