Hawk bullets, in most all calibers, use normal, soft, pure copper jackets, rather than a copper alloy, so they are kind to softer old British double rifle barrel steel. These pure copper jackets, on the Hawk bullets, come in various thicknesses, buyers' choice, for different hunting applications, so one cannot say that the Hawk jackets are "thin," unless one purchased the thin ones they offer, as they also offer quite thick jackets too, for large big boned animals. Hawk custom bullets has a website, so one can read all about any of this, and more. For a good number of years, I have used Hawk bullets in many nitro and black powder express double rifles, with good satisfaction---I highly recommend them.