I have had the opportunity to autopsy about a dozen M1-A CM and Stainless barrels which had between 3K and 4.5K rounds. The classical "mud cracks" had linked together enough to have small chunks gone within 8 inches of the chamber, severe throat errosion, and the stress cracks had penetrated below the surface (metallurgical cross section at high mag) enough for me to be concerened about the continued integrity of the barrel even for "rattle battle". I would never recommend going past 3 K from what I saw if you don't want to test your fate.
Mule