This weak caliber is not really a decent self defense round, even when loaded with slugs. Never use birdshot.
The 20 gauge is an excellent caliber for self defense. It is particularly well suited to those of smaller stature and those that dislike the blast and recoil of the 12 gauge. When compared to a 12 gauge, the 20 gauge delivers 75% of the lead with a recoil that is 40-50% less. This is equivalent to the ballistic force of being hit with two .44 Magnum rounds simultaneously. Reduced recoil of the 20 gauge is conducive to accurate, rapid shots.
For close-in home defense situations, birdshot might work, but its use is not preferred. Recommended shells include #4, BB, or larger hunting loads. Fill the balance of the magazine with #3 buckshot for insurance, due its proven ability to penetrate. Buckshot is preferred over birdshot and slug use in the 20 gauge. For those who insist on slugs, use Federal "Classic" or similar rifled slugs. Effective slug use requires careful aiming with a shotgun equipped with rifle sights. Few 20 gauge shotguns are so equipped.
This round has never gained popularity in the United States. For those who may own a sporting shotgun in this caliber, put it to work as a great self defense weapon.
The 12 gauge shotgun is the most devastating and lethal weapon yet devised for inflicting rack and ruin at close range. A safe bet for ammunition selection is to use the 2-3/4-inch 00 buckshot load. The impact of one of these shot shells is essentially equivalent to getting hit with a nine round burst from a submachine gun.
It is probably a good idea to avoid the 2-3/4- and 3-inch "Magnum" loads. Their brutal recoil makes them a bad choice, and little is gained over the stopping power of standard rounds. Control is important, and follow up shots may be necessary. Standard 12 gauge shotgun shells produce plenty of punishing recoil already.
Some shooters prefer #4 or #1 buckshot over 00 buck. Real world one shot stopping success of the #4 buck is a respectable 81-83%. Data hasn't been collected for the #1 buck, but its performance should be even better.
The one ounce slug, fired from a 2-3/4-inch Federal, Remington, or Winchester shell, has a one shot stopping success of 98%. A deer barrel with rifle sights is the appropriate platform for this round. It is not the best choice for self defense because aiming becomes the critical factor in effective shot placement. The high likelihood of scoring hits, an advantage associated with buckshot loads, is lost. Slugs also have ferocious recoil and tend to over penetrate.
Birdshot is not as lethal as buckshot, even at close range. It may make sense for home or apartment defense where the opportunity exists to injure or kill innocent people behind thin walls in adjacent rooms. For defending a single family home, buffered by land, 00 buck is preferred. The choice for birdshot loads is BB or #4 birdshot. Out to a range of 30 feet or so, birdshot is essentially a solid column of lead pellets. Stopping power may not be sufficient, however, due lack of penetration potential.
At close range, birdshot can destroy a great deal of tissue, producing a gruesome wound. The depth of the injury, however, will likely be six inches or less. This is too shallow to reliably affect an assailant's heart or major cardiovascular blood vessels. Because the wound trauma produced by birdshot is not decisively effective, a quick stop to deadly violence is not guaranteed.
Buckshot loads, on the other hand, will exhibit penetration on the order of 12 inches or so, a depth sufficient to intersect vital blood distribution structures and terminate aggression.
Some misconceptions may exist regarding the spread of shotgun pellets or balls. It is not enough to merely point the shotgun in the general direction of an assailant and let fly. Birdshot or buckshot does not create a huge cone of death and destruction that devastates everything in its path. Rather, for a defense or "riot" shotgun with an 18- to 20-inch cylinder or improved cylinder choked barrel, the pellets will spread out about one inch for every yard of range traveled. Across a large room of 18 feet or so, the spread will only be about 6 inches, a circle as big as a coffee cup saucer. At 50 feet, the spread will only be about 16 inches, the size of a large pizza. It is obvious from these patterns that a shotgun blast will not incapacitate multiple assailants at close range. Each would have to be given "individual attention".
The shotgun must be skillfully aimed and fired. Aiming is just not quite as precise as that required for a handgun or rifle to score multiple hits on an aggressor. The massive firepower of the shotgun will likely produce a favorable outcome in any self defense encounter.