Gauge is irrelevant. What matters is the payload, weight of the shotgun, and fit.
.410 can't do more than 7/8 ounce, but with TSS (tungsten) shells, it's an effective turkey and goose gun at 70 yards... because of the payload.
For those that are not familiar with TSS and what has been going on with turkey hunters for a few years, 7/8 of an ounce of TSS is like 1.4 ounces of lead... at 1100 fps. Except... Tungsten is 1.6 times as dense as lead.. so a number 6 pellet carries the same momentum as a number 4 lead pellet.. and the smaller, denser #6 is ballistically superior and carries it's muzzle velocity farther down range.
Well, now you can use .410 for ducks and geese and turkey... not as a stunt, but as a better payload than a 10 gauge using lead. The .410 is very small, light, easy to carry and the recoil of a 7/8 ounce load is minimal. Put a TSS shell in, you get A+++ across the board, but a D for cost.
Sure, the ammo is more expensive.
Well, if you can hunt geese with a .410, you can do all that with any larger hole. And the diameter of the starting hole has no effect on pattern downrange, this is a well established fact. 7/8 ounces out of a well build .410 is exactly the same quality of pattern you would get from a 10 gauge, if the chokes are swapped to get the same pattern density.
Then there are the new 28 gauge 3" magnums. Sure, there is no SAAMI spec for that, but the world is not centered around America. CIP sets a standard for it, and you can get a new 28 in Italy with a longer chamber.
20 gauge? I shoot 1 1/4 ounces of #5 at pheasants in my 12 gauge, but out of my 20 gauge too. Both going 1300 fps. Granted, the 20 ammo is sometimes harder to find. Mostly because apparently people don't know that #5 is best for wild birds.
Now, late season pheasants, I will grant you that more is better and then the 12 gauge comes into play. But...
My gas operated Fabarm L4S weighs 6.75 pounds. It can shoot 3" hunting loads or 1 ounce target loads, all very soft shooting. There are not many 20 gauges that weigh less, and shooting a 1 3/8 ounce 1440 fps load out of a sub-6 pound gun will set you free, baby! That's a punishing recoil.
Bottom line: the most versatile gauge is 12... because you can always load down to as small as 1/2 ounce.. if you hand load... and go as big as 3" magnums. Because I see no need for 3 1/2 inch.. unless... wait for it...
Unless the US, like Europe, adopts widespread non-toxic shot requirements.
If we're shooting non-tox like steel and bismuth.. we need more room in shells to get the same terminal performance...
12 gauge wins. Big shells, can still be build to light weight to carry.
If light weight, low recoil, and terminal performance better than lead are required.. Tungsten shot means .410 can do it all.
So.. how much are you willing to pay for ammo?
What do you anticipate non-tox regulations will be in the next 20 years?
If every competitor on a clays field all shoot steel shot, scores may go down, but doesn't the most skilled competitor still walk home with the win? Who cares if you shoot skeet with steel, the clay still breaks.
What is available at the store now... will change dramatically when non-tox regulations set in. Just look at what is on the shelf during duck season.
As a hand loader, what's on the shelf isn't really all that interesting anyhow. I can make better at home
things to think about.