It probably wasn't been armor-piercing. Those have been off the market since 1986, I believe. In any case, it is designed for piercing steel plate, like on an light armored vehicle. And you can only get those in military calibers like the 5.56x45 NATO (what the M-16 shoots), the 7.62x51mm NATO (what the old M-14s, FALs, and M60s shoot), and the .30-'06, which was used in both World Wars and in Korea in rifles like the Garand.
All three are popular sporting cartridges. The "ought-six" is the basis for no less than four other hunting rounds and the .308 Winchester (what the 7.62mm NATO is called on the civilian market) is the basis for 2 other cartridges.
Rifles are far more powerful than pistols and the bullets are generally longer and thinner. This makes for excellent aerodynamics, but also more penetration, which is why the use of expanding bullets is mandated for hunting. Even an expanding bullet would penetrate a bulletproof vest, probably coming to rest imbedded in the far side of the armor after passing through the victim.
A bulletproof vest is designed to take pistol ammuntion and maybe buckshot without penetrationg. Rifles, especially deer rifles, are about 6 to ten times as powerful as most common defensive pistol rounds like the 9mm, the .40, and the .45. An army-style flak jacket might stop most midpower rifle bullets, but that is not what the cops use because it is very bulky. Most deer rifles are in the same power range as the .308 Winchester. It does not say what he was shooting with, so I can't give a better analsys.
http://en.wikipedia.org/wiki/Body_armorDuring that famous West Hollywood shootout in 1997, the cops were faced with two shooters wearing several bulletproof vests cut up into head-to-toe body armor. The police pistol rounds and buckshot loads could not penetrate the armor, and that is why today the LAPD cars now also come equipped with civilian-legal AR-15s, what the Army calls the M-16.
http://en.wikipedia.org/wiki/North_Hollywood_shootout