There are materials which can meet or exceed C158 now, but they are just as rare and costly. I have spoken to Carpenter about 9310, which they also manufacture, and he did not consider it to be as good for this particular application (even though it is good for transmission gears, which have a different failure mode).
The AR bolt is the weak link in the rifle, so one should use the best material - and also one should not open the bolt face for 7.62x39mm.

