Countersink Inside a tube

[mklotz]

Another possible technique is a cutter with an inverted cone shape, i.e., the cutting end flares outward from the shaft at the tip, major diameter at the very end of the tool (picture a router dovetail bit, only much smaller). If the diameter of the cutter is small enough to pass through the hole and the angle of the cone matches the required countersink angle, then insert tool through hole and move around in small circle to cut the countersink. Recenter to withdraw from hole.

The geometry works but I sure wouldn't want to be the guy tasked to make it work in practice.