Then some DA will sue somebody for having some standard surplus ball ammo. He'll test fire some through a kevlar vest. "A-HA! GOTCHA!"
Compliant judges and gullible juries will agree: the "armor piercing" ammo falls under the law. Off to jail! Ban the "cop killer" rifle bullets!
"We're going to hammer guns on the anvil of relentless legislative strategy! We're going to beat guns into submission!" ~~Chuck Schumer