Will the bolt for the 7.62x39 work with a 6.5 Grendel barrel in the AR15?

Thanks

No, the headspace for the 6.5 Grendel is set for the extra .011 recess in the face of the bolt. There were some barrels made and headspaced for the standard 7.62x39, but not by Alexander Arms. So if you try to use the 7.62 bolt in an AA rifle the bolt won't close, and if you use an AA bolt in a rifle made for the 7.62 bolt you'll have an extra .011 headspace.

I've used both, but then I set headspace to each individual bolt, so if you're having a barrel chambered and extension intalled, yes, it certainly will work.