(B)ATF(E)ces Online
Under Title 18, UNITED STATES CODE, CHAPTER 44 as amended by Public Law 103-322 The Violent Crime and Law Enforcement Act of 1994 (enacted September 13, 1994) 18 U.S.C. CHAPTER 44 § 921(a)(17)(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.
(C) The term 'armor piercing ammunition' does not include shotgun shot required by Federal or State environmental or game regulations for hunting purposes, a frangible projectile designed for target shooting, a projectile which the Secretary finds is primarily intended to be used for sporting purposes, or any other projectile or projectile core which the Secretary finds is intended to be used for industrial purposes, including a charge used in an oil and gas well perforating device.
A better explanation of the BATFEces deranged version of "the law"...
Fact:
"Armor piercing ammunition" is defined in federal law [18 U.S.C. 921(17)(B)] as "a projectile or projectile core which may be used in a handgun and which is constructed" of various metals harder than lead, or "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." SS-109 bullets used in M855 have a steel tip under the jacket, but they have a lead core.
As if this were not enough BATF has specifically exempted M855/SS-109 along with .30-06 M2 AP.
Straight from the horse's mouth (ATF):
NOTE: THE FOLLOWING CARTRIDGES HAVE BEEN REMOVED FROM THE DEFINITION OF ARMOR PIERCING AMMUNITION:
5.56MM (.223) SS-109 and M855 Ammunition, Identified by a green coating on the projectile tip.
US .30-06 M2 AP, Identified by a black coating on the projectile tip.
It is illegal to "manufacture or sell" armor piercing ammunition (Yes, I am refering to the "Pistol Caliber" ammunition as described in your post.) But, you may possess it and legally shoot it out of a firearm depending on state and local laws.
I know, and work with guys that have thousands upon thousands of rounds of .308 AP ammunition. Specifically, the M2 AP rounds.
BTW- you bring up a very, very good point. There is no regulation on rifle caliber (meaning no able to be used in a handgun) AP ammunition. You can get .30 caliber AP in .300 WM and .338 Lapua, and .50 BMG among other calibers easily.
Mike