I was out at the range on the weekend printing groups with my Tikka T3 6.5x55 Swede (love this thing) and during my 3rd sting I noticed the case pictured below. It still printed it's usual 0.5MOA 5 shot group.
Virgin Lapua brass (neck sized)
Sorry about the poor quality (phone camera) I tried to capture the soot in the extraction groove.
I haven't seen this before and it has me concerned... I'm not sure what the issue could be. If someone could point me in the right direction, it'd be greatly appreciated.