I am relatively new at handloading, but am having excellent results so far. This last batch I made has a strange problem. While shooting, I was having fliers to the right about 3/4". This has been a VERY accurate load, under 1/2" @100yds. I looked at my brass after a few of these fliers, and noticed that there was a carbon streak running all the way down the side from the case mouth to the rim. The whole rim is coated with soot. Somehow gas is escaping. The only thing different with this batch is it was once fired brass out of someone else's rifle. I full length resized, trimmed, and tumbled before loading these. My dies are set to barely bump the shoulder back. I shot 40 rounds and every one off the bullseye had these marks on them. My chamber was clean and dry, my brass was clean and dry. You can see the streaks down the cartridge on the bottom, and all the carbon on the rims. Any insight is appreciated as to the cause of this.
