in KMG there is a lot of confusion on Chakravyuha and Brahmashira weapon. Krishna says only He, Pradyumna & Arjuna knew how to break the Chakravyuha, none else, one can say that Bhishma , Drona and Karna did not knew how to break the Chakravyuha, but then again - was Krishna talking about only the Pandava side? or as a whole?.
The Brahmashira weapon - ( for the info Brahmashira is Brahmastra x 4) -, this weapon is said to be in possession of only three warriors, Drona, Arjuna & Aswathama, so one can say Bhishma and Karna did not have it, but then again - From where Drona get the knowledge of Brahmashira?, if it is from Parshurama, wouldn't Parshurama teach the Brahmashira weapon to Bhishma and his favorite student ( Karna )?, If Parshurama can give his personal weapon and personal bow to Karna, cant he give him the knowledge of Brahamshira?, lots of things to wonder.