I have an M24 that did well on its first outing, considering I've only shot my standard load in it (which was based on what shoots best in my PU sniper). It's a bit frosted and counter bored.
According to anything that I've ever read it doesn't degrade accuracy**, and was done to improve the accuracy of barrels with damaged or worn crowns. Think about it - in a modern rifle you'd cut and recrown the damaged barrel. In a military rifle that uses a socket bayonet you can't really cut down the barrel. Counter boring essentially creates a new crown inside of the barrel.
It's also written in collectors guides that the presence of counter boring should not be a consideration with regards to value.
**With every rifle made shooting differently, the only real way to determine the effects on accuracy of counterboring would be to get a pristine 91/30, accurize it, shoot it, have it counter bored, then re-test. Otherwise you have nothing but anecdotal evidence comparing different rifles that were all built to minute-of-man specs to begin with.
