The 12ga flechette rounds I bought were pathetic. Barely went "pop"!
Interesting use of a Ballistic Products shot cup, but the ribs on the cup take up valuable space inside the hull, thus drastically reducing the payload.
So if you want to shoot a dozen small flechettes at barely enough velocity to pierce paper at 10 yards, then it's money well-spent.
Like others, I've played around with loading my own 12ga flechette rounds. Best results were reasonable velocity (went through a heavy *solid* wood commercial door at 10 yards), but terrible dispersion (six foot circle pattern at that distance!). And very scary internal ballistics.
There's five problems with flechettes from 12ga:
1. Packing them into the hull. If you load half of them point-forward and half point-back, they fill much more efficiently. But the point-back ones are inherently unstable out of the muzzle.
2. Pattern size. Even the point-forward ones will "weathervane" out of the muzzle, where the powder gasses blow past them making the fins work *against* stability.
3. Correct powder load. Even at full capacity, steel flechettes only get you up to 1/2 oz, which is uncharted territory. I ended up with huge amounts of Bullseye trying to get a decent velocity. I still have the LAST hull I fired in this line of experimentation. One side of the hull forward of the powder line was completely vaporized. Scary, scary!
4. Pushing the flechettes down the bore. The flechettes will pierce the bottom of the shot cup as their momentum fights the gas pressure. I put a thin steel disk in the bottom of the shotcup to cure this.
5. Preventing damage to the hull, cup, and barrel. The flechette fins are sharp, and will slice through the hull and cup to scar the barrel. I did it more than once, and recovered sliced hulls. I wrapped a thin steel sleeve inside the shot cup to prevent this.
I've also got some of the flechette rounds Destiny makes for the 40mm grenade launchers. I haven't tried them yet, but they do carry a much heavier payload than a mere 12ga, and could be handy in a home-defense scenario. Of course you'd probably have an attached rifle as a primary weapon...
For the automatic grenade launchers, the M1001 40mm Cannister Cartridge shows what it takes to get the concept working properly.
I've seen the Dragon's Breath rounds in action, and they certainly seem to work. I don't know if I'd shoot one inside a building I cared about, tho...