The parent case of this wildcat is the .300 Winchester Short Mag. All I do is run that brass through the resizing die twice...using a .338" expander button on the depriming stem the first time and a .358" expander button the second time. It is easy. But I can provide brass with the rifle.
Welcome to reloading. I've been at it for over 50 years now. I hardly ever buy factory ammo for any of my guns.
Jerry/AK
