How many rounds can you shoot during a pratice session from an AR15 and not shorten the life of the barrel? I have the new PO stainless 97S and my original BM upper. Both have been broken in, but they really get hot in a short time. With my investment and replacement potential I really worry about damage to these barrels. I understand in a life and death situation that is not a concern, but in practice sessions I do not want to damage them needlessly. I realize this can be subjective but there must be some form of guideline. Thanks