vertical string = unnatual pressure on the stock. My brother in law did that with my Optima after i just shot a 1 1/4" 5 shot group. He put so much pressure by pulling the wrist down tight that his 3 shots were a 8" vertical string.

Different projectiles shoot differently. The Copper series Powerbelts shoot dead center when im sighted in and the Platinum powerbelts shoot 2" to the right.