In level flight if your convergence is set to 400yd and your con is 400yd flying at the same speed as you, your rounds will fall short if you place the pipper on him. How can this be.....50cal takes .49sec to travel 400yds. At 350mph how far does your plane travel in .49sec? 350mph is about 513ft per second, so .49sec is about 200ft that you are missing by.
Haven't you ever wondered why in the drone ring you miss when you simply place the pipper on the drone at range? It's no different than when you set your pipper to the inside of a drone's turn with a bit of elevation to drop the rounds into where the drone will be flying. With a 6 chase it's not as obvious that you have to account for .49sec of travel distance when you shoot.