I purchased a Winchester Mod. 70 African Express in 375 H&H off Gunbroker a little over a year ago. It sat in the safe till recently. A couple of weeks ago I loaded up 10 rounds using new Norma brass and 300 Gr. Hornady DGX bullets (for cheap practice only). I bore sighted the gun at 25 yds. The scope is a 1.5-5 Leupold VX-3. To put it mildly, the groups were terrible. At first I thought it was me or something I was doing wrong, you know, the don't blame your tool sort of thing. I started to reload the once fired cases when I noticed something peculiar. On every piece of brass there was an indentation about 1/8th inch down from the mouth that is about 1/8" wide. Take a look at the attached photos. My question is what could have caused this and can it be corrected?