This is confusing, it doesn't say anything about solid pure copper bullets, but it does have a rule about the weight of the jacket.
Quote from the ATF
" 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.
§ 922(a) It shall be unlawful -- "