With parallel regulation the sights could be counted on at all ranges.

If the bullets shoot into a single group, ie: crossing to impact at a given range, at further ranges, the impacts will be diverging from the line of the sights, shooting further and further apart depending on the range.

In 1850, W. Greener wrote that wrote the bores should be made to converge (cross) at a given range, be it 50 yards or 100yards as the only other alternative was that they diverged and different sighting would have to be made for each barrel when shooting at any range. for all intents and purposes, over normal ranges to shoot together at 50 or 100yards would be perfect for most people as they'd be shooting bore axis apart at 100 or 200yards, THEN divergence would take place further out where few tread.

In 1860, Lt. J. Forsyth wrote that W. Greener's premise that the bores would shoot either crossing or diverging was incorrect.

Forsyth noted that perfect regulation was attained when the impacts of each barrel were perfectly parallel - equal distance apart at all ranges, so that the 'sights' would always give virtually the proper point of impact for each barrel. The centre of the individual barrel groups being only bore axis to bore axis apart, thus no holding off would be necessary no matter the range.

In reality, for the bores to shoot very close at 100 or 200yards would probably suffice today for most people.