I mark my rod to indicate when the ball is seated. But in the course of a day's shooting, since I never clean the barrel from start to finish, I find the mark making its way a little further from the muzzle as the day progresses, even with the same seating pressure. This is apparent mostly in small bored rifles, like my .40's. Though the patch cleans the bore efficiently so that each round gets the same bore condition as the last, the area where the powder rests below the ball continues to get fowling build up throughout the shooting session. As the fowling in this 'chamber' area increases, it leaves less room for the powder, so the powder column rises, as much as 1/4" over the course of the day. I don't mind this phenomenon, or doubt my seating pressure, but this has come with many years of enjoying this great sport.