I have a BlackHole Weaponry barrel and is marked 264LBC 1-9. It is chambered in a 6.5. Grendel 20 inch. I have a 7.62x39 and some people say I can use the bolt off that upper.
What do you think?
You can measure it. Drop an unfired case in the chamber. Take your 7.62x39 bolt and try to close it on the case. If it goes to full battery you have a type 1 chamber. This is most likely what it is anyway, but most were marked that way.