This is why the gun industry begged people not to make home-brew AR15 pistols for years. Once a pistol fires .223, all .223 ammo can be outlawed. The Remington bolt-action pistol and the Thompson-Center Contender both chambered rifle cartridges long before the roll-your-own AR pistols became an issue. There were a few other makes and models that I'm forgetting, too... including one that I believe had a rotary breech that looked like a miniaturized piece of field artillery.
Technically, this justification pre-existed the asinine "cop-killer bullet" ATF regulations.
Rifle cartridges fired from single shot pistols are not “armor piercing” by an ATF ruling. ATF considers single shot pistols to be for “sporting purposes.” Because the M855 projectile contains a steel perpetrator it would have been classified as “armor piercing” like the long gone cheap steel jacketed Chinese 7.62x39 if it hadn't been expressly exempted by law.