The 25x284 is a wildcat cartridge. You will have to make your own brass for it, no one I am aware of manufactures it. I`d go with 6.5x284 breass to work with. The 6.5 will require less neck sizeing and should be easier to do. Hornady, Lapua, and Nosler offer 6.5x284 brass and in a pinch 284 Winchester brass is available with Winchesters headstamp.