Basic question by a guy considering building a Grendel

[VASCAR2]

You can fire form 7.62 X 39 cases to use in a 6.5 Grendel. A friend gave me several mixed 7.62 X 39 brass cases which I fire formed to 6.5 Grendel. I use them for light varmint loads where I might be more apt to loose brass. With the cost of fire forming the first time and the lesser case capacity the FF X 39 cases are of little use to some people. I have several moderate loadings on my fire formed X 39 brass and will continue to use them for plinking or light hunting ammo. I found some Sellier Bellot large rifle primers that fit real snug in PPU and my FF X 39 brass.