The drill too use to pre drill the primer hole is a 19/64 the right tap is 5/16x36 tap is very hard to find.
I looked at Grainger on line and didn't find it by it's self have read on several forms people have found it but is expensive, saw on another gun form don't remember which that they had the same problem as you and used a 1/4x20 tap and his own rod to do the same as the Hornady set up.
Ya it's looking like it's going to be hard to find. I'm here at work as a mechanic in a diaper factory. I thought I'd bring some brass to work and do it here but I can't find any in our tool room or in our parts catalog.
Looks like I'll be sending some to Hornady.
Disclaimer: all the time spent looking for taps and doing home projects is done on breaktime. (Unless the boss is gone.)
Thanks for the link Bigngreen. I just whipped out my card and ordered one. With tax and shipping it came to $32.34. That's a lot for an itty bitty tap but I'm sure sending one out to be done would cost more.