RealClearPolicy wrote and article about the M855 issue, and managed to get this comment from BATFE:
Green-tip rounds were classified as AP [armor piercing] in 1986 because the steel penetrator is what is considered the core. It’s the regulatory process, and everyone can argue semantics and perhaps it’s not written very well, but that is the story behind it. … Having the additional component behind the tip isn’t enough to get it out of AP classification.
Only if you unilaterally rewrite the law, which is what they did. If the law is actually followed, any rounds which contains lead, which M855 does, cannot fall under the definition of armor piercing.
(B) The term “armor piercing ammunition†means—
(i) a projectile or projectile core which may be used in a handgun and which is constructed entirely (excluding the presence of traces of other substances) from one or a combination of tungsten alloys, steel, iron, brass, bronze, beryllium copper, or depleted uranium; or
(ii) a full jacketed projectile larger than .22 caliber designed and intended for use in a handgun and whose jacket has a weight of more than 25 percent of the total weight of the projectile.
So does it fall under subsection (i)? No, because it’s not made entirely from those elements. Does it fall under (ii)? No, unless you count the steel penetrator as part of the jacket, which would be twisting the definition of jacket to an extreme.
h/t to Dave Hardy.