I would make a bushing to center a drill a lot smaller than the bore, and
drill thru. Then go thru with drills of increasing size, relying on the
previous drilled holes to keep things centered. When you get really close,
chances are you will get a broken thin lead sheet out, and you're done. If
you have to clean out the last little bit of lead, I think pouring in hot
solder is a good idea. It will alloy with the lead and flush it out.