Can the variation within a given lot of primers cause vertical stringing? I was using some Fed 210s (standard stuff) and was getting 1/2" horizontal with up to 1.5" vertical at 100 yds. Changed it up to CCI (standard stuff again) and got groups that were almost round. Best group was 0.654". The 210s were an older lot (1999 ) so I am wondering if this is the issue. I'll load up some more and shoot again tomorrow to confirm but just wanted to know if this is a possibility or if I am barking up the wrong tree. Would be great to solve this problem. Thanks.