The easiest way would be to start with 7mmBR brass--providing it is still available.
I made a few 30BR rounds just to see what they would look like loaded with a 168 gr Sierra Match King bullet by chucking the bullet, pointed end down, in my drill press and pressing it into the case. The bullet made a nice tapered expanding button. Bullet and case were well lubed but I did have to wrap a piece of sandpaper around the base of the case for traction and to prevent scratches, and use pliers to twist the case free.