Have you dealt with 6262 before? It is a free-turning alloy and in my experience with turning and milling the chips come of in nice small flakes. But what I have found with drilling is that these small flakes jam up in the flutes of the drill and don't clear away. I haven't drilled that diameter that deep, smallest I did was 1/8", but if you haven't already tried it I suggest short pecks (1/2D?) and plenty of coolant.