I shoot a HiWall in 40-82 and ran into a similar problem. Gun was originally intended for black powder and relied on the obturation to get the bullet to fill the bore. In my rifle, if I use a bullet that matches the bore, there is too little room in the neck for normal brass thickness. I turned some necks with a forester tool and shoot bullets close to bore size. Black powder still is best though.... with real black powder and soft lead bullets it will shoot 3" or so at 100 yards.