It will of course vary a bit from maker to maker & type to type, but on a typical 12ga double the bbls will converge at an approximate rate of .011" per inch. On a 30" bbl'd gun the difference in width of the C/L would be about .330" or a bit over 5/16". This will result in an extension of the bores axis crossing around 2 yds (6 feet) in front of the muzzles & they will then have a separation of about 15" at 40 yds. The torque / twist of the gun upon firing does indeed cause the right bbl to impact further right than its static point of aim & the left bbl to impact further left, thus they tend to impact the same point at practical shooting distances. "IF" the bbls came to a parallel condition the right bbl would impact to the right of point of aim & the left bbl to the left.
