Effective range for what animal or bird?
Effective range for what shot size?
Effective range for what bore size?
Effective range or maximum range?
Effective range of lead, bismuth, tungsten or iron shot?

When considering range, one must also be aware of the drop of the shot- how high to aim to get a charge of rapidly slowing pellets to high with the centre of the target. At close range, this is not worth considering, but if shooting, say a 10 bore on geese at 75 to 80 yards with #2 LEAD shot, one must allow a good 1 1/2' to 2' for drop depending on the load.

Prove that to yourself on a patterning board - as well, to shoot geese reliably at that range, one must have a 95% (40yd) or better patterning gun to shoot effectively at those long ranges, btw. He must also use lead or tungsten pellets. No factory ammo will do it that I know of.

The absolute effective range is highly manipulated by the choke in the gun, the shape of the choke in the gun & how that gun handles a particular ctg, shot size, wad column and loading.

Barrel length has very little to do with the gun's effective range when using modern powders and components.

Back before chokes were invented, in the mid to 3/4 19th century, Ely sold ctgs. which contained a wad in it's base, with the shot contained inside a copper wire cage covered with paper of different colours. The different colours were labeled as to the effective range of each ctg. These were designed for use in the cylinder bores of the day. It is written, that the green (I believe) ctg. had a maximum range on ducks and geese to 100yards or more, but was effective for wolves and deer to 50 or 60 yards from a 14 bore SXS caplock shotgun. These range far exceeds our very best choked guns today, with the very best loadings in 12 bore by close to or over 40yards on ducks and geese.

Unfortunately, the Ely ctgs. are no longer made. History tells us they didn't always work, but were popular with the wealthy shooters of the day.

The normal effective range of a cylinder bored shotgun of the day, with loose shot and wads was a mere 35 yards.

Pattern density and shot size deterimes a gun's effective range.

Today, most waterfowl shot with non-toxic shot are killed at closer then 50yards. That would make that the maximum range - I would assume, the effective range would be from 0 to that range at which you can kill the game without wounding.

I hope this helps. I know of no shotgun made today that could even remotely be considered for ethicaly shooting AT any game at 100ayrds, let alone 300. This, is spite of many attempts by hutners every fall - some even to trying buckshot - sky busters we call them.