The gun is a new (test fired 20Jul12) BH convertible 45ACP / 45LC. The 45ACP cylinder throats were, uniformly, less than 0.451. The 45LC cylinder had 4 throats less than 0.451 and two at about 0.451. The attached photo shows a Rainier plated 0.451 SWC with the ACP (left) and LC (right) cylinders. (For the purists: yes, I did measure the Rainier (Starrett 233) and it is .451.) Manson
sell the Manson 0.4525 throat reamer and pilots for about $130 plus shipping.
Rather than buying, I rented a reamer and pilots from 4-D Products
. Rental cost was $35, ship to me $8.25, return ship $5.25.
Reaming requires use of a cutting fluid (e.g. Tap Magic) and a handle to hold the reamer. The reamer's handle shaft is cylindrical with one narrow flat. A "T-handle" tap wrench
has insufficient grip, so I chucked the reamer in a drill press and turned the spindle by hand.
The reamer is self-feeding, requiring no down pressure. Time required to ream the two cylinders was about an hour for me, but I do tend to go slowly. Results were good … a .452 will drop through all chambers under it's own weight.
For additional background, see here
. Note that the T-handle shown probably won't work.