From the website: http://www.backwoodshome.com/articles2/ayoob110.html

This article, and several other statements or mentions in other writings by Ayoob give most of my reasoning for preferring the 20 gauge pump for the basic staple for home defense situations.

Shotguns

Historically, the .410 shotgun has been a common "starter gun" for shooters, especially younger ones. It is the smallest and lightest "gauge" available. However, many of us think it's too small.

Interchangeable choke tubes, seen here at muzzle of 20 gauge shotgun, give more versatility.

These little shells just don't hold enough lead to always get the job done. Personally, I see the .410 as a close-range squirrel and rabbit gun. Noted the great Frank C. Barnes in his authoritative text Cartridges of the World, the rifled slug load in the .410 is "not adequate for deer," though he believed it would get business done with a coyote or bobcat.

On the other end of the spectrum is the 12 gauge, by far the most popular backwoods home shotgun. The reason for that popularity is its versatility: the big two and three quarter inch 12-gauge shell holds an ounce to an ounce and a quarter of lead, and that's just in its standard loading, before you get into the Magnum options. A bigger spread of birdshot gives you a better chance of cleanly killing game birds; a bigger spread of buckshot is more devastating in the anti-personnel context; and a bigger chunk of lead (weighing 400 to 500 grains, at 1400 to 1600 feet per second) is more decisive on deer and close-range bear.

However, the 12 gauge is also infamous for its recoil with anything but a light trap or skeet load. Many a lad has been knocked on his butt firing his first 12 gauge. In an adult lifetime of teaching police, I've met a lot of macho man cops who were scared to death of the 12 gauge "riot gun's" kick, and a couple of female officers who were crippled for life by it through shoulder damage sustained from the constant hammering of intensive training fire. A full load 12 gauge in a light pump gun kicks about like a .375 Magnum elephant rifle.

No shotgun is simpler to unload than this NEF single barrel, available in several gauges.

To split the difference, I think the best bet for a light-kicking but hard-hitting shotgun is the 20 gauge. You can virtually duplicate 12-gauge performance with some three-inch Magnum 20 gauge shells, but for most needs, the standard two and three-quarter inch 20 gauge shell will get the job done.

In a standard rifled slug load for deer, the 20 gauge's projectile is approximately .62 caliber, weighs five-eighths of an ounce, and runs at a speed of 1400 foot-seconds or greater. That's roughly the equivalent of two .44 Magnums at once. Will it transform Bambi swiftly into venison? Oh, yes.

In a buckshot load, the standard for 20 gauge is #3 buck, which consists of twenty pellets, each a quarter inch in diameter. Among common 12-gauge shells, the closest would be the so called "urban police load," #4 buck. This consists of 27 pellets, each .23 caliber. Velocity is roughly the same, whether twelve or twenty gauge.

For home defense purposes, let's say two evil twin men are kicking down the front door, announcing their intent to murder every man, woman, and child in the house. Spouse A fires a round of 12-gauge #4 buckshot and sends 27 .23'' diameter pellets into the chest of the first evil twin. Spouse B unleashes a single round of 20 gauge #3 buckshot, blasting 20 .25" diameter pellets at approximately the same velocity into the thorax of the second evil twin, with all the projectiles penetrating to approximately the same depth.

Who's going to know the difference? Only the medical examiner, and then only after he meticulously counts the white spots on the X-rays, or tallies the number of pellets removed from each corpse.

Author pops a shell into a .410 single barrel. He considers that round too underpowered to be useful for much.

Birdshot? Only in long range waterfowl hunting does the 20 gauge seem to really give away much to the bigger 12, and then it is largely because non-lead pellets required for lo, these many years to keep lead out of the wetlands are not very efficient penetrators. This means that with, say, steel shot, reduced momentum and reduced penetration need to be made up for with more pellets striking the bird.

For upland game such as pheasant or partridge, or for hunting small game such as wild rabbits for the family stewpot, shotgun authority Francis E. Sell proved in the 1950s that the 20 gauge loses precious little compared to the 12 gauge, and there is the three-inch Magnum 20 gauge shell available for those who want a bit more punch. Lighter and much faster to handle than most 12-bores, the 20 improves wing-shooting skill for a great many people. My significant other, a tough little critter who stands barely five feet tall, refuses to shoot my 12 gauges anymore, but is hell on wheels with her Remington 1100 Youth Model 20 gauge semiautomatic.

If the user(s) can benefit from a lighter, easier-to-carry gun that kicks less, the 20 gauge makes huge sense. In similar size shotguns, its recoil will be barely over half that of the 12 gauge. The 20 is available in all action types: semiautomatic and pump, side by side double barrel or over and under, and low-price bolt actions and single shots.