Think his allen mags tapper to .960 at the muzzle, not sure about what his threads are...His email is on his posts here,,,,I think he just got back from hunting and he is still answering tons of emails...send him an email, he will answer..He answered mine, but do know he's busy as a one armed paper hanger
I emailed Kirby a while back asking the same question. This was his response:
For the large 5 port brake, anything in the 0.950" to 1.050" range works very well.
My large 5 port PKs are $255 shipped to you or your smith. Just send payment the address below in my signature. They are in stock ready to ship. These brakes need to be indexed, reamed 20 thou over bullet diameter and surface finished to your desired finish. Easily an hour or less work. --