John,
Jim Stroh at Alpha Precision has a technique for removing the thread choke that he calls Taylor Throating. I've had him do it to one of my Rugers, and it made a remarkable difference with cast bullets.
I'm currently firelapping a Vaquero in an attempt to remove a slight thread choke. I've fired 20 rounds so far, and it is working slowly. I expect that 50 rounds may remove my constriction, but it's not quite as tight as yours.
Beartooth Bullets sells a "Technical Manual" for $10-$12 that's a real good read on preparing handguns and rifles for cast bullet accuracy. The book covers firelapping in great detail. Even if you don't like the idea of firelapping, it's an interesting book for just a few bucks.
The Blade