I start by rough facing the end of the barrel. This isn't the final end pass however; that'll come when I set the cylinder gap.
A depth mic is used to measure the front of the frame to the cylinder face. This one mic'd 0.641". I'll turn the shank to that exact length, 0.690" o.d.
Ruger frames are 24 tpi. Since Blackhawk barrel inlets are countersunk, there's no need for a relief cut before the shoulder. The depth of compound feed for a single form vee tool is 0.036". Total depth on the compound then equals ~0.072". This is a rough approximation though. Ultimately, the frame dictates how deep I'll have to go to obtain nice, tight mating.
Using a cutting bit, I lathed off the first two threads. Otherwise, these would show through the frame window.
Testing the frame on the threaded barrel - this one turned out perfect. It was snug but had no bind or tight spots.
The forcing cone - there are a lot of theories around which style works best. The angle isn't an exact science, as long as it isn't too shallow or too steep. 5 to 15 degree cones work, but we normally use 11 degrees. I don't like deep forcing cones and I'm not a fan of throating. My throats are properly indicated and the cylinder is rigid in battery. Big forcing cones and free-bore only do two things well: 1) They correct poor alignment, and 2) They eliminate choke. My dad summed it up best. If the cylinder-to-bore alignment is dead nuts and the cylinder doesn't move, get the bullet into the lands ASAP.
The compound feed is moved to 11 degrees and a boring bar is mounted.
A close-up of the shallow 11-degree cone prior to polishing. It was hard to take a crisp picture of it due to poor lighting, metal bits, and oil.
The barrel is screwed into the frame and as expected, the shank just touched the cylinder face. I removed it and faced 0.002" off the end. The barrel was re-installed, verifying a 0.002" cylinder gap.
Two things to note:
1) Since I haven't drilled the ejector hole or installed a sight, I'm not bound by thread timing. From what I understand, Ruger adds the front sight and drills the housing hole prior to final installation. If the thread timing isn't perfect, they're chasing TDC on the sight when torqueing. This is why barrel choke is so common in today's Blackhawks.
2) Step 19's barrel indication – again, if this is off, the threading and cone will show runout to the throat. Line-boring and line-indexing only work if the barrel is installed with precision alignment between the bore's axis and the frame inlet.