I spoke with David from T/C this morning and had a long discussion about what he thought might have been the cause of the crack in my barrel.
After elimiting all the usual causes, he came to two different conclusions.
#1 it could have been build up of 777 and some of it piled up in front of the bullet. He said 777 will leave a clear coating of build up in the barrel after about 10-15 shoots. the gun will still load and shoot well but the build up is still there.

#2 it could have been bad tooling or metal fatigue

either way I'm glad to get the barrel back.