I just noticed flechettes came up here and since I posted a reply earlier in legal on them I will repost almost word for word here.
They are very neat. Basicly they are little more than nails, except instead of a round head they have slots or fins up the side. In fact I imagine short nails with the head cut into fins would function exactly the same. The fins have to be proportional to the size of the projectile, so smaller nails would be easier to stabilize. Duplex nails would have 2 heads to turn into fins giving even more stability. Although I imagine any nail would work even without alteration since the head would have the highest drag keeping it pointed relatively rearward and the point relatively forward, altered ones would just work better cutting down on total drag.
A denser material than steel but still hard would have greater performance.
They are excellent for penetration, and would indead be "armor piercing" if talking about soft body armor, especialy larger ones. However the same thing that makes them great for penetration makes them horrible for terminal performance. The thin profile of something like a nail means it just applies most of its energy in a very small point causing little damage to surrounding tissue. This means it takes a lot of them, and they are not immediately incapacitating. Although for head shots the very good penetration and high hit probability would probably make them quite effective since the brain does not usualy take a lot of punishment.
If you think of ammunition and total surface area as well as depth penetrated in total volume of tissue destruction all the points of the flechettes are going to total less than the total diameter of the shotgun bore. Yet Buckshot or even any shot is going to total more surface area than the diameter of the bore.
Think of it this way for example. #1 buck is judged to go the suitable 12" of penetration. Each pellet is .30" in diameter. That means 16x.30" is 4.8" multiplied by another 12 inches of penetration gives you 57.6" total area damaged in the target at maximum possible potential. 00 buck is a little less, but not far behind giving up a little total area for longer range and more penetration.
Now take the points of the Flechettes. They are going to all total under the (lets assume 12 gauge) .729 bore diameter munus some for the shotcup. Since a torso is not usualy much wider than 12 inches (with many less) extra penetration is usualy not going to amount to additional damage. So taking the shotcup into consideration and the fact that the fins are going to reduce the total number that can fit (although if of shorter length some can be positioned higher up than others with the fins interlocking the shaft of others to pack the most possible) you are going to have less than .70X (multiplied by) the depth it penetrates of tissue damage. There is a maximum of penetration that matters though after which it will not be in the target. So if 12" is your maximum depth then the total tissue damage would be 8.4". If your target is thicker than 12" you can factor in those additional inches for a higher amount of tissue damage.
So you have 57.6" with #1 buckshot, and around 8.4" with flechette of tissue destriction. It would be higher if you count the total depth penetrated by the flechette, but since the target is only so thick, that is usualy irrelevant. Quite a difference in total damage.
Now there would be some yaw, and the fins would add a little damage, but not much.
So the total amount of tissue destruction will be less than with buckshot.
For birdshot type uses flechettes would be far too dangerous as they would rain down like spikes. So discussion of them for that purpose is not even necessary.
So more damage would be done by shot that goes to suitable depth. Then of course the individual diameter of each would is also important as there is a minimum for effectiveness. Otherwise hypordermic shots into the heart would be lethal, yet they are not.
There is also a cut off where thinner no longer freely leaks blood, meaning its wound channel is not effective.
So in both hunting and defensive use the diameter of each projectile is important as is the total momentum of each. So this would limit you to fewer yet larger flechettes with a total impacting surface area of less than the bore of the shotgun.
As pointed out they are illegal in several states including CA. In much of the nation they are not.
Unless penetration is more important than tissue damage they are usualy not a good choice.
Most commercial ones are made of steel and steel and other hard ammunition is very ricochet prone so use caution.
The only time they might be more suitable for a job is if you need that extra penetration. Shooting an animal through heavy brush for example.
So flechettes and irregular shaped shot as I was talking about earlier in this post are almost complete opposites.
The irregular shot would do more damage than spherical shot, and the flechettes would do even less damage.