This is solid, if not classic information:
http://www.firearmstactical.com/briefs10.htm.
Aside from initial patterning, if you want to be competent with your weapon it requires practice. Not just for the primary owner, but for anyone who could potentially be using the shotgun. As a result, target loads are used for practice far more often than buckshot loads.
Number 1 buckshot is readily available from a number of sources in any configuration you prefer. All you have to do is call Clint at Stars & Stripes and he can provide what you'd like:
http://www.starsandstripesammo.com/index2.html. You can take that as a strong recommendation.
Very few people bother to pattern at all, much less do any penetration testing. To most, buckshot is buckshot and that's as far as it goes.
The average self-defense shot is fired within 15 feet. Defensive performance is a whole different school of thought from offensive performance. Using buckshot at 25 yards is considered murder, not "defense" in many areas. Regardless, you will likely be arrested and jailed. You can also expect to be sued.
Any obsession with over-penetration is silly. If you are forced to use a firearm to protect yourself, when there is no other reasonable alternative, it means there is an immediate threat to your life. The threat needs to be eliminated-- that's all there is.
Calibrated ballistic gelatin is a good medium for comparison. Dr. Fackler, MacPherson, etc., have long shown that. At the same time, it is basic to wounding ballistics that all wounds are unique, no two wound cavities are identical. Ballistic gel has no breathing or circulation, has no vital organs, does not wear leather jackets, is not changed by drugs, emotion. Ballistic gelatin does not move, throw its arms up, or shoot back. As ballistic gelatin is not alive, it is hard to kill it. No need to stop it as it isn't moving in the first place.
You are counting on one shot to eliminate the threat. It had better be a good one. Focusing on overpenetration is off-topic; it could get you killed. The only reason to fire in the first place is because you have no alternative. If you were going to a gunfight, you'd bring a rifle. That isn't the idea.
The idea is to remove a clear, immediate threat to you and your family, at close range, instantly. It is fight or flee and in your own home fleeing is often not possible. If a shot is fired and you have not eliminated the threat-- you have failed. That failure is far, far more dangerous to you, your family, and what might be on the other side of a wall than any over-penetration theory.
To assure adequate penetration in all situations, you absolutely must use projectiles that will over-penetrate. Collision dynamics on bone, layers of clothing, fat, etc., are not precise nor predictable. Too little penetration will get you killed.
It is defense, defense of your own life, your family's life. So, you choose a firearm and load
as if your familiy's life hangs in the balance-- that is the whole idea.
