That is a lot of interesting info zoogster.
But your saying I cannot make the bullets out of titanium? Or gold?
No you can, but the gold would obviously be far more expensive, and the titanium would be so light weight that it would not hold momentum and be unsuitable as a bullet.
Mainly for cost purposes it means you cannot use iron/steel which is a small fraction of the price, put in a cheap jacket.
Do keep in mind this is the federal law, state laws can be even more restrictive.
Is brass unacceptable as a bullet?
Brass is illegal as a bullet for any cartridge for which a handgun has been made, including oddities like a Contender and Encore.
Here is the relevent part of the federal law:
The definition of AP ammo is at 18 USC sec. 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.