777 is a crud ring powder, if there is any of the crud left in the barrel it will be right above where the bullet seats and will affect your seating pressure and can cause vertical stringing. The barrel heating or uneven pressure on the for arm screws can also do this. I put a couple of washers between the forearm and the barrel and check the torque on mine. Of course I also use Blackhorn 209 which eliminates the cleaning between shots so is much easier for me to get to group. The Shock Wave shoots really good for me with 110 to 120 gr of BH, so I usually use 115 gr.