Not a whole lot to know about a countersink (I have been using one for my military brass primer pockets and cleaning up case mouths for nearly 25 years). The primer crimps displace a small amount of case material at the top of the primer pocket. A countersink (I've been using a 60 degree high speed steel) cuts the displaced metal and "opens up" the primer pocket allowing for a clean/easy insertion of a new primer. Sometimes I chuck one in my hand drill or power screwdriver and it takes only a second to cut the offending crimp out...
Never tried the dedicated primer pocket "swage" or "reamer" for primer pockets 'cause it weren't necessary and I had a few countersinks handy. Works very well for me and my .223/5.56, 45 ACP, and 9mm brass.
My Anchor is holding fast!