A standard .223 AR-15 round will go through a quarter in steel plate like it was butter.

[thatdemguy]

6. velocity

Look at it this way try having a yugo hit you at 5 mph, it might hurt, it will probably knock you down, now imagine that yugo at 55 mph.

Both bullets are normally lead with a copper coating or jacket.

As for body armor, any rifle round will go thru a police type vest and some pistol rounds will too. Apparently one of the pistol rounds used by russia in ww2 will go thru most police vests. Even some rifles/ammo will go thru armor plates designed for normal rifle rounds. From what I have read it all about speed, the faster the bullet the more stuff it can go thru armor wise.