Flechettes....bad idea
The military may, at one time, have experimented with flechette ammunition for 12-gauge shotguns. The fact that the military now uses buckshot should tell you something about how effective it was.
There are four main problems with loading the commonly available flechette into a shotgun and trying to use it for offensive/defensive purposes.
1) Flechettes are light, there were between 19 and 20 in each of the 10 cut apart shotshells we evaluated. That's not a lot of weight per projectile.
2) Flechettes are pointy. Icepicks are pointy, but few people rely on icepicks for combat.
3) Flechettes have fins. You wouldn't think of it as a drawback until you combine that with 1) and 2) above. You wind up with a projectile that doesn't have a lot of momentum to begin with and then has little fins on the back that act like very effective brakes to keep the flechette from penetrating very far.
4) Flechettes, to gain whatever weight advantage they can are packed with half of the payload pointing forward and half to the rear. As soon as those rearward flechettes reach open air they lose any stability they may have had in the barrel and fly off ..... well pretty much like any unaerodynamic, non spinning projectile. Picture trying to throw a broadhead arrow backwards and you'll quickly get the idea.
How horrible are they? In the testing we did, at 15 yards the pattern was 6-7 feet in diameter. A 6-foot diameter pattern has over 4,000 square inches. To hit that you have at most 20, inch-long flechettes that will penetrate about an inch and a quarter...at most. In reality, the great majority of those flechettes will hit broadside, without a bit of penetration and probably not even causing a welt on bare skin.
If I had to face someone carrying a shotgun loaded with flechettes or someone carrying a shotgun loaded with birdshot..... there's not much choice in who I'd rather be charging.
If anyone is curious to see the test and the pictures, head over to GeorgiaPacking.Org and do a search for flechettes.