You can use 6.5 Grendel, 7.62x39mm brass to make 6mm ARC brass. 7.62x39mm brass will be out there for as long as 5.56 NATO brass. So, im not that concerned about brass shortages. As I see it, 6mm ARC is more of a replacement for AR10 in 308, but can do double duty and also replace 5.56 NATO.