I have some insight into IIIA. When the first designs came out they had no problem stopping a heavy 44MAG round, but the problem was that the energy transferred behind the armor was too great. Similarly, a IIIA might be able to stop some smaller rifle rounds, but it could still leave the wearer mortally wounded and definitely incapacitated. The design modifications really drove up the cost. IIA and III is a good compromise for a majority of rounds fired at police, but for the situation you are planning, who knows.

I think a big reason why armor is effective for military and police is having a medic and evac at hand. From the guys I've talked to, getting hit in armor is not easy to shake off and keep going. I don't know how useful it would be without a support team around, but than again, better than nothing.