Well, that is kinda of a tough question; there is not even a good rule of thumb. We see plenty of ported guns that "prove" that a hole or even a series of holes in a shotgun barrel don't necessarily lead to failure. Pressure guns had holes drilled and tapped in the chamber area and didn't blow. So, it probably is going to take a severely damaged barrel to literally blow, but, fingers and innocent bystanders being involved, if in doubt, consult a pro.

DDA