It's time for me to add the non-mechanical rifle or ammo cause of first round's way-out shot placement. I had this situation when I started high power competition.
After getting into position and loading the first round, new (inexperienced?) shooters oft times pull the trigger and/or hold the rifle different for the first shot. It goes to some other place other than where the center of all the following shots go.
When I laid down on the 600 or 1000 yard line with my Garand and its sights were set to the best zero established for the load used, that first shot was high and left. At 600 yards 'twas about a foot out from dead center at 10:30 o'clock. Subsequent shots were well centered. So, I just adjusted my second sighter shot (when allowed) to strike there if the first one didn't. That second shot went to call; center if so called. It took a couple years to get my mind controlling my body muscles to shoot that first round exactly the same as all those that followed. It was an emotional high to finally be able to adjust my second sighter to strike dead center instead of out in the 8 ring.
300 yard rapid fire from prone was a different issue. I always (yes, every time, both with a Garand and bolt action match rifle) had to get my sighters at 10 o'clock in thel 10-ring; about 2" high and 3" left of dead center. When I stood up, loaded the first 2 or 5 rounds then laid down and fired them, they went center. Reloading the last 8 or 5 rounds also went center. Never did get that corrected. But I've learned over the years that others have the same problem.
US Navy Distinguished Marksman Badge 153
Former USA Palma Team Member
NRA High Power Long Range High Master
NRA Smallbore Prone Master