I've done this before, but it's been a lot of years. I can't remember what I did? I seem to recall using my drill press, but how to hold the cases?
I've tried a regular countersink bit, and it leaves some of the pockets too tight, still. I found a Hornady pocket reamer in my kit, and it works better. But it grabs so tight, the cases spin in my fingers.
I rigged up a system to stick the cases in my case gauge, then stick a lyman 223 expander rod into the mouth to keep it from spinning. Then I can ream the pockets with the Hornady reamer bit in a drill.
Is there a better way? All my case holding/locking stuff seems to lock onto the rim, covering the primer pocket.