Interesting observations. Type III anodizing is difficult to measure using the Rockwell tests since it is such a thin coating. It is typically applied around 0.002 thick. Half of that .002 is build-up on the surface and the other half of the .002 is penetration of the underlying surface. Type III anodizing is more of an abrasion resistant coating rather than a hardening agent due to it's thickness. Sealing further reduces the hardness of this coating. If the substrate is being dented by shell casings something isn't right. Either it isn't hardened properly or it's not 7075-T6 no matter what they're marketing it as. 260 Brass, typically called cartridge brass, is approximately B77 Rockewll. 7075-T6 is approximately B87.