Looks like this guy had the same problem. This was not you was it. Looks like he's in Florida.
All AR ( abrasion resisting ) plate is not the same. Comes in many hardnesses.
AR w/5% up to 14% Manganese ( this one will get very hard very fast as the mangenese is high for this exact purpose ie impact resistance)
ThewThe AR550 and 600 are somewhat brittle ( if you beat on them when they are cold they can crack etc.
These are considered somewhat machinable until you get to about 550 BHN.
Armor plates are like the above but may have extra elements added to make them more tough and more shatter resistant. They also have balistic tests done to them. The info says the material is weldable which may help. since we dont know how hard it is except that it is probably about as hard as your cobalt bit, I think the diamond drill suggested above may be your best bet.