It's not so much a particular source that it was read from , but rather information that has been accrued over my years shooting a 12 ga. I figure that just as most firearms ,even of the same make and model, tend to have preferences for certain types of ammo, so it goes for the ole shotgun. From experience and discussions with people over that last 20 years. It averages out that a cylinder bore barrel will shoot 1 inch per yard, if it is a good one, and it also helps if it is a more modern one.
In general most shotguns of the riot gun type would actually shoot a little worse than 1" per yard, but if you got one that did you considered yourself lucky. Today they call a barrel cylinder bore when it isn't really a cyl. bore. Typically they even have some constriction and it varys between manufacturers. In all actualality they tend to be more of a fixed modified choke. The real problem is related to the nature of a shotgun barrel being different internally than others. The amount of constriction in one barrel may throw a tighter pattern than another with the same choke, as the amount of constriction isn't the same for every manufacturer. You might get a good 18 pattern with cyl. bore on a new shotgun and then use the same model in say a 1976 cyl. bore riot gun and get 24 at 20 yards. Then you put a modified choke in the older gun and throw a 16 pattern with the choke in it. Then comes back boring in the barrels. Just as was said earlier more shot usually means a little larger pattern in the same shotgun, simply due to more shot getting in each others way to get through that slight bit of constriction provided by a choke system. What back boring will do, especially for a 3.5 inch shotshell is even out the spread and keep it a little tighter than it would without it. The problem with back boring is that it usually prevents the use of slugs, the only reason I can imagine that causes this little drawback is a pressure spike when it hits the chokes and that the thinner walls at the beginning of the barrel can't take it, therefore you are advised, by manufacturers, to not shot slugs out of a back bored barrel. Then there comes a point where your choke can have too much restriction and you patterns can get worse from it being too tight, not to mention in some cases it can be bad for the choke, barrel and potentially the gun and operator.
Suffice it to say, this is the reason people are often told to take a new shotgun and pattern it with different ammuntion and chokes. Find the best that meets your particular need and go with it. Another thing to consider is that people used to and in some cases still do cut a barrel down to the 18 inch riot gun length and then expect them to shoot as tight as a modern made barrel. When their new "custom shotgun" doesn't they go about blaming somthing for the problem, other than considering it is because they now have a true cylinder bore without the countours of a purpose built barrel on their new "tacticool" "shotty". For the best patterns buy a barrel and forget using a hack saw.
As the shotgun becomes more refined by experience and technology, expect them to get a little better than you see them today. Still, you should always know that the closer you get to optimum the more difficult it becomes to eek out more performance. Just like an old muscle car. They can get more powerful and have better handling, they still won't match a modern perfomance car, as the modern tech and knowledge gained from years past allow the purpose built machine to far exceed the origals in the envelope they were designed within. That old musle car can match you if the driver is more skilled and they sure do have a personalty, whereas the new stuff is almost sterile in nature.
There really is an art in the making of a shotgun barrel and getting the proper internal demensions in sync to have a much better patterning gun. Alot of things change and many stay the same. If you do load your own, putting buffer in with the buck shot helps even out the patterns as well. The reason has to do with less deformation of the pellets and hence less scatter and randomness in that spread.
Get your shotgun, load up on the ammo and pattern that sucker. Get to know what she likes and dislikes. Just like a good woman, she'll make you better when you really understand what she wants to be at her best, in turn making you better.
The organic nature of shotgunning has a method all it's own. For the most part this is where the saying, you don't aim a shotgun, you point it comes from. Granted this is more true of a custom bird gun, usually one of those ultra high dollar doubles, that seem to be outrageous in price come in. They are custom fitted and allow the shooter and shotgun to act as one, aiming only slows the process of engaging moving targets. To some degree aiming is a gun a conscious effort and it contadictory to typical shotgun useage. In a sense thinking about shooting and shooting are not conducive to great shotgunning. Even will all that said. It all comes back to the intended purpose of the shotgun you are going to use. A bird gun, slug gun, deer gun, and riot/ combat shotgun all have a slightly different set of standards to be met that will make them good at their job. Get what feels good to you and that which suits your needs the best.
Like most of you guys out there, I tend to like the riot gun for home defence and general purpose use and don't need a field gun for anything other than the occassional clays session or hunting bambi now an again. So it goes back to all that jazz with the lights, sights and the potential fights your partner may be needed in. The best thing to do is shoot, shoot, shoot. At the end of the day you'll find you know what you can, cannot, should and should not do with that boomstick.