"Normally" the barrels of a double were straightened prior to joining. I joining they were set to converge from breech to muzzle. This would ordinarily mean the barrels did not touch for any distance behind the muzzles. Having a desire to check this several years ago, but not wanting to saw off my barrels I did the following;
I marked the barrels at several increments beginning at the muzzles & going back a few inches. At each mark, I measured the OD of each barrel & then the width across both barrels. On every gun I checked the width across the barrel was greater than the sum of the ODs of the two barrels. This means that each one that I checked would have had a gap between the barrels at any point behind the muzzles.
As I learned though, many, many years ago in Elementary English Grammer "There are Exceptions to All Rules"
