25 Grendel or 25 BR

[grayfox]

For optimum projectile/brass arrangement, it's going to require a COL that no AR15 magazine or receiver set can handle currently.

For loading long in Grendel, it won't require much at all to get neck tension. Bushing dies will work.

Mocking it up in Grendel, it already has some neck tension in Lapua factory virgin brass.

So are we talking use of a windowed mag? or will this have to be single loaded?
Or re-barrel a howa mini and use its mag length?