Self explanatory. I discovered later the tap could have been driven by a 5mm socket. I measured the depth of the primer pocket, and sure I turned the equivalent number of threads so as not to hit the decapping pin.

Case is tapped, about 4 threads worth. I chose an 8mm thread for this, because I thought a larger diameter would be less likely to strip the threads on extraction. I guestimated, correctly this time.

After drilling and tapping, the decapping pin is unharmed.