Dented Case Mouths

I am loading 6.5g of Unique in 45 Starline brass w/ 230g Fed. Hydra-Shocks. They are 875fps through the chrono. My gun is a WWI Colt 1911 mfg. in 1918. The gun, though it is old, does not have much wear. I am just beginning to load for this gun. It has been in a safe since the end of WWI. I have changed the spring to a 20# variable Wolf spring. The mags are new My question is why is my brass dented on the case mouths?

david