If handgun bullets could go through body armor, then the bullets would be illegal or the body armor would be improved. And even if there was a good armor piercing round made for civilians, it would be horrible against unarmored threats.
Now, if someone is using home made armor out of just regular steel plate then almost anything should go through it. I have a spinning 3/8ths thick steel target that's ratted for a .44 Magnum. A 33-30 put a hole in it with regular Federal 150 grain soft points at 80 yards. I can only imagine what a .243 would do. But unless someone is an idiot and thinks they can put regular steel plate, this probably would do nothing more than bruise someone with real armor.