Roy;
Don't want to seem overly picky but the first part of your statement doesn't jive with the 2nd part. The bbls themselves were Not set to converge at 40yds, they were set to have the patterns impact together at 40yds. As has been well pointed out by several in this long thread the bbl's axis themselves are set to converge very close to 2 yds (between 6&7 feet).

Miller/TN
I Didn't Say Everything I Said, Yogi Berra