1) The 1963 introduction of the Winchester Mark V collar to prevent bore scrub reduced the inside diameter of the hull. So as reported by Jack O'Connor in The Shotgun Book
1970 printing, page 303:
00B was reduced from .330 to .323
#1B was reduced from .298 to .289
#4B was reduced from .240 to .233
Recently the size of current Federal brand 00B and #1B were noted at .323"/50 grains and .286"/33grains, respectively. See:
2) In the same tome O'Connor reports his pattern tests indicated, with a given shotgun barrel, the effective range would be extended from 30 to 50%.
Federal introduced its Premium line of buckshot in 1984, advertising patterns under 15 inches at 40 yards from full choke shotguns Federal Hunter's Journal
1986. This level of performance was confirmed in my own pattern testing.
3) Steel shot wads introduced high capacity, static, thick wall wads to the market. This in turn led to the development of the Dixie Tri-Ball buckshot round by Dixie Slugs co. This buckshot load contains three 320 grain, hard cast, .60" pellets, buffered and contained in a steel shot type wad. I have personally fired many patterns with this load measuring under 5 inches at 40 yards with some less than 3 inches from a Remington 870 Express with a Briley extended full choke tube.
The images attached show a 40 yard Tri-Ball pattern on an 8" bullseye target and a comparison shot of Dixie Tri-Ball pellets next to an equivalent weight of 00B.