I've been reloading for my S&W M&P 15T, 5.56/.223.
I was out trying several different loads yesterday, shooting 10 shot groups, two diff loads per session. I was picking up my brass and found this one;
I can't say I've seen one like that and have no idea what caused it. The load was Varget, Sierra 55 or 60 gr bullet, once fired Winchester case, and under max by a couple grains. Any thoughts?