In my previous article, I briefly overviewed the Soviet 6mm program.This program was built upon a series of studies and a research and development initiative focused on sub-caliber fin-stabilized discarding sabot (FSDS) flechette ammunition, which took place from 1960 to 1981.The motivation behind this research largely stemmed from the US Special Purpose Individual Weapon (SPIW) program, which ran from the 1950s to the 1970s.In the mid-1960s, engineers at NII-61, now known as TsNIITochMash, developed sub-caliber FSDS ammunition.
The legislation already exists.
18 U.S.C., § 921(a)(17)(B):
It would be difficult to produce flechettes not caught by that existing definition.
Putting aside legality and assuming that is overcome, it would still be an uphill fight in the market. I would posit that the majority of people buying handguns do not highly weight armor penetration as a factor. Any company going into flechettes as an armor piercing solution would have to eat all the R&D costs to get flechettes that sufficiently penetrate armor from a pistol length barrel, produce or partner with an ammunition company to produce this new and expensive ammunition that only works for one brand on the market, produce and sell handguns for this. All hoping to recoup costs by catering to the subset of the market that is willing to pay significantly higher prices just for armor penetration.
The only viable customer for a company to pitch flechettes to is the military, and once more- all of the rifle attempts have had insurmountable problems. Investing in flechette handguns simply wouldn’t be worth the time, as handguns are considered of low importance to small arms and current calibers work well enough. Even armor piercing 5.7mm never caught on widely for military adoption.