Sorry about the angry children that have a habit of hijacking legitimate questions. We understand that they have issues, unfortunately, its a waste of time for those who don't have such issues and who are interested in learning.
What you have is a metallurgical stress fracture and it has to do with an impurity in the steel. It happens from time to time and is virtually impossible to avoid it due to the smelting process.
I would also suggest that you stick to c158 or 9130 bolts. Both are very good and have very similar properties as I would caution against using bolts with harder materials as they are dissimilar (harder) then the barrel extension and can cause damage/premature wear of the extension itself.
It's just unfortunate that you got a bad one but fortunate that you got it from a company that stands by their products.
Good luck!