yeah, no worries! disagree or not, I'm just thankful that everyone has been civil and no hard feelings on this end for sure.
This is how we all learn, good healthy debate and different points to consider; none of us are right all the time.
It would be fun to test out, may be worth taking a cheap single barrel and chopping it down inch by inch and shooting a box of shells on each length to get an accurate result.
The book excerpt I posted does say they tested with normal/legal lengths so not sure what they consider the shortest 'normal' length. I'm assuming 18" but the 12 and 14" breaching 870's we have at work are legal (to a select few of course) also as they're military and LE weapons so who knows; the book never stated the shortest length tested so don't really know.
I'd be curious to start with say, a 30" barrel and take it down 2" at a time until you hit about 18", then take it down inch by inch, all cylinder bore with the same exact shells.
A good fresh grease board and a fixed camera on a tripod to take photos of each shot on the board, flip through them at the end to see what the pattern does as barrel is removed.
maybe a used NEF shotgun or something, they're like $100? course you'd have to destroy the thing at the end as it would be illegal with a 1" barrel!