Just for a ROM (rough order of magnitude) check, say the measured convergence of the barrels is hypothetically .020". In 10 inches downrange that's .200". In 100 inches (about 8 1/3 feet or less than 3 yrds) downrange that's 2". In 1000 inches (83 feet or 28 yrds)that's 20 inches. Given that the barrels are only about an inch apart (centerline) or more like 3/4" at the muzzles, the gun should theoretically shoot crossed patterns about 19" apart at their centers at 28 yrds, or about a foot crossed patterns at the 16 yrd line.
I chose .020 since I know my little guns are less than this and the big bores are more. But this should illustrate that there are dynamics involved that simple trigonometry doesn't account for.
