All center-fire rifle ammunition is “armor piercing.”
Even my blunt nosed 30-30 will go through 3/16” steel
Almost went through a 1/4” swinging target.
"(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."
It's a legal definition, not necessarily a functional or common sense definition. You'll note the references to "use in a handgun", and "constructed entirely" of steel, brass, etc. - large caliber 'solids' (example: https://www.midwayusa.com/product/197389/norma-solid-ammunition-375-h-and-h-magnum-300-grain-solid-box-of-10 ) intended for dangerous game may be constructed entirely of brass, but are not considered armor piercing ammo, because they're not intended for use in a handgun.
I'm not sure if "armor piercing" rifle ammunition is currently prohibited in any significant way; please provide details if you have them. The 'green tip' 5.56 ammo may not qualify as "armor piercing" because it is not constructed entirely of any of the listed substances (the steel penetrator sits on top of a lead core).
There are licensing requirements for certain commercial ammunition manufacturing activities; maybe the gentleman ran afoul of one or more of those...